Trump vows to sue Cruz over birthplace if 'false ads' stay up

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Trump Threatens to Sue Cruz

WASHINGTON (Reuters) -- U.S. Republican presidential front-runner Donald Trump blasted close rival Ted Cruz on Monday, threatening to file a lawsuit challenging his eligibility for the White House if the Texas senator does not take down his "false ads."

The New York billionaire, whose campaign has long been littered with insults, called Cruz "totally unstable" and said he is "the single biggest liar I've ever come across, in politics or otherwise, and I have seen some of the best of them."

Speaking at a news conference in South Carolina, Trump said he could "fight back" by bringing a lawsuit against Cruz over the fact that he was born in Canada, which Trump argues makes him ineligible to become president.

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Trump vows to sue Cruz over birthplace if 'false ads' stay up
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidates (L-R) Donald Trump and Sen. Ted Cruz (R-TX) participate in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
Republican presidential candidate, businessman Donald Trump speaks with the moderators as Republican presidential candidate, Sen. Ted Cruz, R-Texas, looks on during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, businessman Donald Trump, left, speaks as Republican presidential candidate, Sen. Ted Cruz, R-Texas, speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican presidential candidate, Sen. Ted Cruz, R-Texas, speaks as Republican presidential candidate, Sen. Marco Rubio, R-Fla., left, and Republican presidential candidate, businessman Donald Trump gestures during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, businessman Donald Trump speaks as Republican presidential candidate, Sen. Ted Cruz, R-Texas, looks on during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican presidential candidate, Sen. Ted Cruz, R-Texas, right, speaks as Republican presidential candidate, businessman Donald Trump looks on during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican Presidential candidate businessman Donald Trump (L) speaks next to Texas Senator Ted Cruz during the Republican Presidential debate sponsored by Fox Business and the Republican National Committee at the North Charleston Coliseum and Performing Arts Center in Charleston, South Carolina on January 14, 2016. / AFP / TIMOTHY A. CLARY (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidates (L-R) Donald Trump and Sen. Ted Cruz (R-TX) participate in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
2016 Republican presidential candidates, from left, Senator Marco Rubio, a Republican from Florida, Donald Trump, president and chief executive of Trump Organization Inc., and Senator Ted Cruz, a Republican from Texas, arrive for the Republican presidential candidate debate at the North Charleston Coliseum in North Charleston, South Carolina, U.S., on Thursday, Jan. 14, 2016. The sixth Republican debate comes at a time with less than three weeks before Iowa caucus-goers cast the first votes of the 2016 presidential election on February 1. Photographer: Andrew Harrer/Bloomberg via Getty Images
LAS VEGAS, NV - DECEMBER 15: Republican presidential candidates Donald Trump (L) and Sen. Ted Cruz (R-TX), interact at the conclusion of the CNN republican presidential debate at The Venetian Las Vegas on December 15, 2015 in Las Vegas, Nevada. Thirteen Republican presidential candidates are participating in the fifth set of Republican presidential debates. (Photo by Justin Sullivan/Getty Images)
Ted Cruz, right, makes a point as Donald Trump reacts during the CNN Republican presidential debate at the Venetian Hotel & Casino on Tuesday, Dec. 15, 2015, in Las Vegas. (AP Photo/John Locher)
LAS VEGAS, NV - DECEMBER 15: Republican presidential candidate U.S. Sen. Ted Cruz (R-TX) speaks during the CNN Republican presidential debate on December 15, 2015 in Las Vegas, Nevada. This is the last GOP debate of the year, with U.S. Sen. Ted Cruz (R-TX) gaining in the polls in Iowa and other early voting states and Donald Trump rising in national polls. (Photo by Justin Sullivan/Getty Images)
Republican presidential candidate businessman Donald Trump (L) pat the back of Texas Sen. Ted Cruz during the Republican Presidential Debate, hosted by CNN, at The Venetian Las Vegas on December 15, 2015 in Las Vegas, Nevada. AFP PHOTO/ ROBYN BECK / AFP / ROBYN BECK (Photo credit should read ROBYN BECK/AFP/Getty Images)
Donald Trump, left, and Ted Cruz joke about remarks Cruz has made about Trump's temperament during the CNN Republican presidential debate at the Venetian Hotel & Casino on Tuesday, Dec. 15, 2015, in Las Vegas. (AP Photo/John Locher)
MILWAUKEE, WI - NOVEMBER 10: Republican presidential candidate Donald Trump (L) and Ben Carson (C) looks on as U.S. Sen. Ted Cruz (R-TX) speaks during the Republican Presidential Debate sponsored by Fox Business and the Wall Street Journal at the Milwaukee Theatre on November 10, 2015 in Milwaukee, Wisconsin. The fourth Republican debate is held in two parts, one main debate for the top eight candidates, and another for four other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
Donald Trump, left, and Ted Cruz shakes hands at the start of the CNN Republican presidential debate at the Venetian Hotel & Casino on Tuesday, Dec. 15, 2015, in Las Vegas. (AP Photo/Mark J. Terrill)
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"If he doesn't take down his false ads and retract his lies, I will do so immediately," he said.

Cruz scored a key early victory in the Republican race to pick a nominee for the Nov. 8 presidential election when he won the Iowa caucuses earlier this month, and has been attacking Trump on a range of issues.

Trump said the Republican National Committee should intervene to stop Cruz from misrepresenting his views and policies on issues such as abortion, gun rights, healthcare and potential Supreme Court nominees.

He said he had signed a pledge with the RNC agreeing not to run a third-party candidacy, and to support the eventual Republican nominee, and the RNC should do its bit to keep the campaign fair.

"The bottom line is the RNC is controlled by the establishment ... and special interests," Trump told reporters. "I signed a pledge, but it's a double-edged pledge. As far as I'm concerned, they're in default on their pledge."

Trump's remarks, days ahead of Saturday's Republican primary in South Carolina, came after increasingly bitter exchanges between the two candidates in recent weeks. Cruz has hammered Trump on key issues, and said on Saturday that if elected he would appoint liberals to the Supreme Court.

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Trump vows to sue Cruz over birthplace if 'false ads' stay up
FORT WORTH, TX - FEBRUARY 26: Republican presidential candidate Donald Trump speaks at a rally at the Fort Worth Convention Center on February 26, 2016 in Fort Worth, Texas. Trump is campaigning in Texas, days ahead of the Super Tuesday primary. (Photo by Tom Pennington/Getty Images)
Republican presidential candidate Donald Trump gestures as he speaks at a campaign rally Friday, Feb. 5, 2016, in Florence, S.C. (AP Photo/John Bazemore)
Republican presidential candidate Donald Trump speaks during a campaign stop Wednesday, Feb. 3, 2016, in Little Rock, Ark. (AP Photo/Tony Gutierrez)
Republican presidential candidate Donald Trump holds up a child he pulled from the crowd as he arrives to speak at a campaign rally in New Orleans, Friday, March 4, 2016. (AP Photo/Gerald Herbert)
TOPSHOT - Republican presidential candidate Donald Trump gestures during a campaign rally in North Charleston, South Carolina, February 19, 2016. / AFP / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images)
Republican presidential candidate businessman Donald Trump gestures during the Republican Presidential Debate, hosted by CNN, at The Venetian Las Vegas on December 15, 2015 in Las Vegas, Nevada. AFP PHOTO/ ROBYN BECK / AFP / ROBYN BECK (Photo credit should read ROBYN BECK/AFP/Getty Images)
Republican presidential candidate Donald Trump speaks during a campaign event at Plymouth State University Sunday, Feb. 7, 2016, in Plymouth, N.H. (AP Photo/David Goldman)
Pastor Joshua Nink, right, prays for Republican presidential candidate Donald Trump, as wife, Melania, left, watches after a Sunday service at First Christian Church Sunday, Jan. 31, 2016, in Council Bluffs, Iowa. (AP Photo/Jae C. Hong)
Republican presidential candidate Donald Trump speaks during a campaign rally in North Charleston, South Carolina, February 19, 2016. / AFP / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images)
Republican presidential candidate, businessman Donald Trump blows a kiss after speaking at his caucus night rally, Monday, Feb. 1, 2016, in West Des Moines, Iowa. (AP Photo/Jae C. Hong)
Republican presidential candidate Donald Trump speaks during a campaign rally at the Pensacola Bay Center in Pensacola, Fla., Wednesday, Jan. 13, 2016. (AP Photo/Michael Snyder)
Republican presidential candidate Donald Trump speaks during a campaign stop before next months earliest in the nation presidential primary, Monday, Jan. 11, 2016, in Windham,NH (AP Photo/Jim Cole)
RENO, NV - JANUARY 10: Republican presidential candidate Donald Trump holds a campaign rally at the Reno Event Center on January 10, 2016 in Reno, Nevada. Donald Trump continued to raise doubts Sunday about rival Ted Cruzs eligibility for the presidency, saying Republicans will risk losing a lawsuitand potentially the nations highest officeif they nominate Cruz as their candidate. (Photos by Charles Ommanney/The Washington Post via Getty Images)
Republican presidential candidate Donald Trump pops through a doorway as he arrives to address a gathering during a campaign stop at a Rotary Club luncheon in Manchester, N.H., Monday, Feb. 8, 2016. (AP Photo/Charles Krupa)
Republican presidential candidate, businessman Donald Trump speaks during a rally at the Greater Columbus Convention Center in Columbus, Ohio, Monday, Nov. 23, 2015. (AP Photo/Paul Vernon)
MESA, AZ - DECEMBER 16: Republican presidential candidate Donald Trump speaks to guest gathered during a campaign event at the International Air Response facility on December 16, 2015 in Mesa, Arizona. Trump is in Arizona the day after the Republican Presidential Debate hosted by CNN in Las Vegas, Nevada. (Photo by Ralph Freso/Getty Images)
Republican Candidate Donald Trump arrives to speaks to the press with Rev. Darrell Scott(R), senior pastor of the New Spirit Revival Center in Cleveland Heights after meetings with prominent African American clerics at Trump Tower in New York November 30 ,2015. AFP PHOTO / TIMOTHY A. CLARY / AFP / TIMOTHY A. CLARY (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)
COLUMBUS, OH - NOVEMBER 23: Republican presidential candidate Donald Trump addresses supporters during a campaign rally at the Greater Columbus Convention Center on November 23, 2015 in Columbus, Ohio. Trump spoke about immigration and Obamacare, among other topics, to around 14,000 supporters at the event. (Photo by Ty Wright/Getty Images)
DORAL, FL - OCTOBER 23: Republican presidential candidate Donald J. Trump attends a campaigns rally In Florida at the Trump National Doral on October 23, 2015 in Doral, Florida. Trump leads most polls in the race for the Republican presidential nomination. (Photo by Johnny Louis/FilmMagic)
COLUMBIA, SC - SEPTEMBER 23: Republican presidential candidate Donald Trump leaves a campaign event September 23, 2015 in Columbia, South Carolina. Earlier today, Trump tweeted 'FoxNews has been treating me very unfairly & I have therefore decided that I won't be doing any more Fox shows for the foreseeable future.' (Photo by Sean Rayford/Getty Images)
Republican presidential candidate, businessman Donald Trump reacts during the CNN Republican presidential debate at the Ronald Reagan Presidential Library and Museum, Wednesday, Sept. 16, 2015, in Simi Valley, Calif. (AP Photo/Mark J. Terrill)
NEW YORK, NY - JULY 22: Donald Trump greets supporters, tourists and the curious after taping an interview with Anderson Cooper at a Trump owned building in mid-town Manhattan on July 22, 2015 in New York City. Trump, who is running for president on a Republican ticket, has come under intensifying criticism for his behavior on the campaign trail. The billionaire's most recent comments on Senator John McCain's war record in Vietnam have resulted in almost universal criticism from fellow candidates. (Photo by Spencer Platt/Getty Images)
Republican presidential candidate Donald Trump leaves for lunch after being summoned for jury duty in New York, Monday, Aug. 17, 2015. Trump was due to report for jury duty Monday in Manhattan. The front-runner said last week before a rally in New Hampshire that he would willingly take a break from the campaign trail to answer the summons. (AP Photo/Seth Wenig)
Donald Trump, president and chief executive of Trump Organization Inc. and 2016 Republican presidential candidate, left, looks on as Sam Clovis, newly appointed national co-chairman of Trumps campaign, speaks during a news conference ahead of a rally at Grand River Center in Dubuque, Iowa, U.S., on Tuesday, Aug. 25, 2015. President Barack Obama's top business ambassador dismissed Trump's call for a wall along the Mexico border, saying the U.S. is focused instead on expanding business with one of its biggest trade partners. Photographer: Daniel Acker/Bloomberg via Getty Images
BILOXI, MS - JANUARY 02: Republican presidential frontrunner Donald Trump pauses with supporters after speaking at the Mississippi Coast Coliseum on January 2, 2016 in Biloxi, Mississippi. Trump, who has strong support from Southern voters, spoke to thousands in the small Mississippi city on the Gulf of Mexico. Trump continues to split the GOP establishment with his populist and controversial views on immigration, muslims and some of his recent comments on women. (Photo by Spencer Platt/Getty Images)
Republican presidential candidate Donald Trump pulls his hair back to show that it is not a toupee while speaking during a rally at the TD Convention Center, Thursday, Aug. 27, 2015, in Greenville, S.C. Trump says his trademark hairdo is for real. He told 1,800 people in South Carolina Thursday: "It's my hair ... I swear." (AP Photo/Richard Shiro)
Supporters reach to great Republican presidential hopeful Donald Trump as he leaves a brief stop to speak to supporters and the media in Laredo, Texas, Thursday, July 23, 2015. Trump predicted Hispanics would love him, because as president he said he'd grab jobs back from overseas and give more opportunity to those who live in the U.S. legally. (AP Photo/LM Otero)
Republican presidential candidate Donald Trump gestures during the first Republican presidential debate at the Quicken Loans Arena Thursday, Aug. 6, 2015, in Cleveland. (AP Photo/John Minchillo)
Republican presidential hopeful Donald Trump remarks about Texas Gov. Rick Perry's glasses at his South Carolina campaign rally in Bluffton, S.C., Tuesday, July 21, 2015. (AP Photo/Stephen B. Morton)
LAREDO, TEXAS - JULY 23: Republican Presidential candidate and business mogul Donald Trump talks to the media at a press conference during his trip to the border on July 23, 2015 in Laredo, Texas. Trump's recent comments, calling some immigrants from Mexico as drug traffickers and rapists, have stirred up reactions on both sides of the aisle. Although fellow Republican presidential candidate Rick Perry has denounced Trump's comments and his campaign in general, U.S. Senator from Texas Ted-Cruz has so far refused to bash his fellow Republican nominee. (Photo by Matthew Busch/Getty Images)
Republican presidential candidate Donald Trump responds to former Arkansas Gov. Mike Huckabee's closing remarks during the first Republican presidential debate at the Quicken Loans Arena Thursday, Aug. 6, 2015, in Cleveland. (AP Photo/Andrew Harnik)
DES MOINES, IA - AUGUST 15: Republican presidential candidate Donald Trump eats a pork chop on a stick and gives a thumbs up sign to fairgoers at the Iowa State Fair on August 15, 2015 in Des Moines, Iowa. Presidential candidates are addressing attendees at the Iowa State Fair on the Des Moines Register Presidential Soapbox stage and touring the fairgrounds. The State Fair runs through August 23. (Photo by Win McNamee/Getty Images)
CLEVELAND, OH - AUGUST 06: Republican presidential candidates Donald Trump (R) and Wisconsin Gov. Scott Walker participate in the first prime-time presidential debate hosted by FOX News and Facebook at the Quicken Loans Arena August 6, 2015 in Cleveland, Ohio. The top-ten GOP candidates were selected to participate in the debate based on their rank in an average of the five most recent national political polls. (Photo by Chip Somodevilla/Getty Images)
OSKALOOSA, IA - JULY 25: Republican presidential hopeful businessman Donald Trump speaks to guests gathered for a rally on July 25, 2015 in Oskaloosa, Iowa. During his last visit to the state Trump sparked controversy when he said Senator John McCain (R-AZ), a former POW, was not a war hero. (Photo by Scott Olson/Getty Images)
Republican presidential candidate, businessman Donald Trump, speaks during the Values Voter Summit, held by the Family Research Council Action, Friday, Sept. 25, 2015, in Washington. ( AP Photo/Jose Luis Magana)
Republican presidential candidate, businessman Donald Trump addresses supporters at a campaign rally, Monday, Dec. 21, 2015, in Grand Rapids, Mich. (AP Photo/Carlos Osorio)
TURNBURRY, SCOTLAND - JUNE 08: Donald Trump Visits Turnberry Golf Club, after its $10 Million refurbishment, on June 8, 2015 in Turnberry, Scotland. (Photo by Ian MacNicol/Getty Images)
DERRY, NH - AUGUST 19: U.S. Republican presidential candidate Donald Trump hugs an American flag as he takes the stage for a town hall meeting in Derry, New Hampshire, August 19, 2015. (Keith Bedford/The Boston Globe via Getty Images)
DES MOINES, IOWA - AUGUST 15: Donald Trump at the Iowa State Fair on August 14, 2015 in Des Moines, Iowa. The Iowa State Fair is one of the oldest and largest agricultural and industrial expositions in the United States. The annual fair, the largest event in Iowa, attracts over a million visitors each year. The fair runs through August 23. After a brief walk around its time for Mr Trump to board his club cart again and leave the fair. (Photos by Charles Ommanney/The Washington Post via Getty Images)
AYR, SCOTLAND - JULY 30: Republican Presidential Candidate Donald Trump visits his Scottish golf course Turnberry on July 30, 2015 in Ayr, Scotland. Donald Trump answered questions from the media at a press conference. (Photo by Jeff J Mitchell/Getty Images)
TURNBURRY, SCOTLAND - JUNE 08: Donald Trump visits Turnberry Golf Club, after its $10 Million refurbishment on June 8, 2015 in Turnberry, Scotland. (Photo by Ian MacNicol/Getty Images)
Donald Trump, president and chief executive of Trump Organization Inc. and 2016 U.S. presidential candidate, gestures while speaking during The Family Leadership Summit in Ames, Iowa, U.S., on Saturday, July 18, 2015. The sponsor, The FAMiLY LEADER, is a 'pro-family, pro-marriage, pro-life organization which champions the principle that God is the ultimate leader of the family.' Photographer: Daniel Acker/Bloomberg via Getty Images
PASADENA, CA - JANUARY 16: Donald Trump arrives at NBCUniversal's 2015 Winter TCA Tour - Day 2 at The Langham Huntington Hotel and Spa on January 16, 2015 in Pasadena, California. (Photo by Angela Weiss/Getty Images)
Donald Trump, president and chief executive officer of Trump Organization Inc. and 2016 Republican presidential candidate, pauses while speaking during a news conference at Trump Towers in New York, U.S., on Thursday, Sept. 3, 2015. 'I have signed the pledge,' he said, referring to a document stating that he would not run as an independent candidate in the event that he does not win the Republican nomination. Photographer: Victor J. Blue/Bloomberg via Getty Images
US tycoon Donald Trump arrives to speak at the annual Conservative Political Action Conference (CPAC) at National Harbor, Maryland, outside Washington, on February 27, 2015. AFP PHOTO/NICHOLAS KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)
EDISON, NJ - AUGUST 30: A golf fan takes a 'selfie' with presidential candidate Donald Trump during the final round of The Barclays at Plainfield Country Club on August 30, 2015 in Edison, New Jersey. (Photo by Ross Kinnaird/Getty Images)
ABERDEEN, SCOTLAND - JULY 10: Donald Trump plays a round of golf after the opening of The Trump International Golf Links Course on July 10, 2012 in Balmedie, Scotland. The controversial £100m course opens to the public on Sunday July 15. Further plans to build hotels and homes on the site have been put on hold until a decision has been made on the building of an offshore windfarm nearby. (Photo by Ian MacNicol/Getty Images)
BIRCH RUN, MI - AUGUST 11: Republican presidential candidate Donald Trump speaks at a press conference before delivering the keynote address at the Genesee and Saginaw Republican Party Lincoln Day Event August 11, 2015 in Birch Run, Michigan. This is Trump's first campaign event since his Republican debate last week. (Photo by Bill Pugliano/Getty Images)
BOCA RATON, FL - APRIL 16: Billionaire Donald Trump speaks to a crowd at the 2011 Palm Beach County Tax Day Tea Party April 16, 2011 at Sanborn Square in Boca Raton, Florida. Trump is considering a bid for the 2012 precidency and is expected to announce his running in the coming weeks. (Photo by John W. Adkisson/Getty Images)
Republican presidential candidate Donald Trump speaks during a campaign rally Saturday, Dec. 5, 2015, in Davenport, Iowa. (AP Photo/Charlie Neibergall)
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Trump has long said that Cruz may not be constitutionally qualified to serve as president because he was born in Canada. The U.S. Constitution says only "natural born" citizens can become president of the United States.

Born in Calgary, Alberta, to a U.S. citizen mother and a Cuban father, Cruz has accused Trump of bringing up his birthplace simply because Cruz is leading some polls.

Last month, Cruz said Trump, who led the movement questioning whether the Hawaiian-born President Barack Obama was really from the United States, had asked his lawyers to look into the issue of Cruz's birth in September and concluded there were no issues.

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