Donald Trump took to the airwaves Thursday with a barrage of name-calling in response to news that the Republican Party's last presidential nominee, Mitt Romney, was trying to torpedo the billionaire real-estate developer's chances in this year's contest.
"Mitt Romney is a stiff," Trump said on NBC's "Today Show." "He got killed, he got decimated in the election."
Looks like two-time failed candidate Mitt Romney is going to be telling Republicans how to get elected. Not a good messenger!
Romney is planning a speech later Thursday in a bid to dislodge Trump from leading the party's presidential nominating race, branding the New York mogul as untrustworthy and saying he'd be a boon to Democratic candidate Hillary Clinton.
"Donald Trump is a phony, a fraud," Romney will say later Thursday at the University of Utah, according to a transcript provided to Bloomberg News by a person familiar with his remarks. "He's playing the American public for suckers."
The attempt by the party's last nominee to take down his presumptive successor is an unprecedented escalation in Republican infighting. The criticism marks the bluntest attempt so far by the Republican establishment to slow Trump's momentum after his victories on Tuesday in the single biggest day of voting in the Republican race.
Responding Thursday on MSNBC, Trump said Romney was "a disaster" who failed to inspire voters in 2012. "He was a catastrophe," Trump said on MSNBC. "He ran one of the worst campaigns, as you know, in presidential history."
See more of Romney's past campaigns for president:
Mitt Romney's campaigns for president
Romney tries to slow Trump's march, drawing billionaire's wrath
**FILE** Massachusetts Gov. Mitt Romney speaks at a rally on the steps of the Statehouse in Boston, in this Sunday, Nov. 19, 2006 file photo, Romney's political record is relatively brief - four years as Massachusetts governor and two statewide campaigns - but that's enough for Brian Camenker to write a 28-page report that portrays Romney as sympathetic to gay rights and sexual behavior that clashes with his burnished image as a defender of traditional values. (AP Photo/Chitose Suzuki, File)
Former Massachusetts Gov. Mitt Romney shakes hands with first graders from Mead Hall Episcopal School during a campaign swing through South Carolina as he stops to be the Keynote speaker at a Rotary club meeting Monday, Jan. 29, 2007, in Aiken, S.C. (AP Photo/Mary Ann Chastain)
Republican Mitt Romney meets with the media after addressing the Detroit Economic Club in Detroit, Wednesday, Feb. 7, 2007. Romney used the first major policy speech of his fledgling presidential campaign to promote permanent tax cuts, portable health insurance and free and open markets. (AP Photo/Carlos Osorio)
Republican presidential candidate and former Massachusetts governor Mitt Romney signs a poster for Alex Stroman, of Chester, S.C. Wednesday, Feb. 14, 2007, at Seawell's in Columbia, S.C. Romney visited South Carolina as part of his official campaign kickoff tour following his announcement Tuesday that he's seeking the Republican presidential nomination. (AP Photo/Perry Baker)
Republican presidential candidate and former Massachusetts governor Mitt Romney holds one his granddaughter, Mia, 2, after speaking in Columbia, S.C. Wednesday, Feb. 14, 2007, as part of his official campaign kickoff tour. Romney visited South Carolina following his announcement Tuesday that he's seeking the Republican presidential nomination. (AP Photo/Perry Baker)
Republican presidential hopeful Mitt Romney talks to the nation's first presidential primary voters during a campaign stop at the Opera House in Derry, N.H., Tuesday, April 3, 2007.(AP Photo/Jim Cole)
Republican presidential hopeful, former Massachusetts Gov. Mitt Romney speaks to a potential voter on a cellphone at his campaign headquarters in Boston Wednesday, May 16, 2007, one day after the latest GOP presidential debate, and amid a daylong effort by his campaign to enlist 24,000 new supporters. (AP Photo/Elise Amendola)
Republican presidential hopeful Mitt Romney shakes hands during a campaign stop at Moulton's Market in Amherst, N.H., Wednesday, Aug. 1, 2007.(AP Photo/Jim Cole)
Presidential hopeful and former Massachusetts Gov. Mitt Romney, left, kisses 6-month-old Bennett Covington as he poses for a picture with the baby and his mother, Liz Covington, Thursday, September 27, 2007 , during a campaign stop at an IHOP in Sacramento, Calif. (AP Rich Pedroncelli)
Republican presidential hopeful former Massachussets Gov. Mitt Romney speaks to local residents during a campaign stop Saturday, Dec. 15, 2007, in Humboldt, Iowa. (AP Photo/Charlie Neibergall)
Republican presidential hopeful, former Massachusetts Gov. Mitt Romney, shakes hands with supporters at a campaign stop in Las Vegas, Nev., Thursday, Jan. 17, 2008. (AP Photo/LM Otero)
Republican presidential hopeful, former Massachusetts Gov. Mitt Romney, waves to supports during a primary day campaign rally in Tampa, Fla., Tuesday, Jan. 29, 2008. (AP Photo/LM Otero)
Republican presidential candidate, former Massachusetts Gov. Mitt Romney shakes hands during a campaign rally, Friday, Sept. 7, 2012, in Orange City, Iowa. (AP Photo/Evan Vucci)
A woman working in the room where Republican presidential candidate and former Massachusetts Gov. Mitt Romney will speak election night is silhouetted against a lighted campaign sign near the stage at the Boston Convention Center in Boston, Monday, Nov. 5, 2012. (AP Photo/Stephan Savoia)
Republican presidential candidate, former Massachusetts Gov. Mitt Romney speaks at a Florida campaign rally at Orlando Sanford International Airport, in Sanford, Fla., Monday, Nov. 5, 2012. (AP Photo/Charles Dharapak)
Republican presidential candidate and former Massachusetts Gov. Mitt Romney with his wife Ann greet supporters during a campaign rally at the International Exhibition Center in Cleveland, Sunday Nov. 4, 2012. (AP Photo/Jerome Delay)
Republican presidential candidate, former Massachusetts Gov. Mitt Romney speaks during a campaign event at the Newport News International Airport, Sunday, Nov. 4, 2012, in Newport News, Va. (AP Photo/David Goldman)
Republican presidential candidate and former Massachusetts Gov. Mitt Romney reaches out to shake hands with supporters as he campaigns at Colorado Springs Municipal Airport in Colorado Springs, Col., Saturday, Nov. 3, 2012. (AP Photo/Charles Dharapak)
Republican presidential candidate and former Massachusetts Gov. Mitt Romney carries Levi Vandenberg, five months, of Dover, N.H., after he picked him up from his mother and continued to walk down a ropeline of supporterrs as he campaigns at Portsmouth International Airport, in Newington, N.H., Saturday, Nov. 3, 2012. He later returned the baby to his mother. Ann Romney is seen at left. (AP Photo/Charles Dharapak)
Republican presidential candidate, former Massachusetts Gov. Mitt Romney talks with former Florida Gov. Jeb Bush as they fly on his campaign plane to Miami Fla., Wednesday, Oct. 31, 2012. (AP Photo/Charles Dharapak)
Republican presidential candidate and former Massachusetts Gov. Mitt Romney greets supporters as he campaigns at the Pensacola Civic Center in Pensacola, Fla., Saturday, Oct. 27, 2012. (AP Photo/Charles Dharapak)
Republican presidential candidate, former Massachusetts Gov. Mitt Romney, left, and vice presidential running mate Rep. Paul Ryan, R-Wis., wave to the crowd as fireworks go off during a campaign rally on Friday, Oct. 19, 2012 in Daytona Beach, Fla. (AP Photo/ Evan Vucci)
President Barack Obama, right, and Republican presidential nominee Mitt Romney shake hands following their third presidential debate at Lynn University, Monday, Oct. 22, 2012, in Boca Raton, Fla. (AP Photo/David Goldman)
Republican presidential candidate, former Massachusetts Gov. Mitt Romney shakes hands during a campaign event at Watson Truck and Supply, Thursday, Aug. 23, 2012, in Hobbs, N.M. (AP Photo/Evan Vucci)
Republican presidential candidate, former Massachusetts Gov. Mitt Romney pauses during a campaign event at Horizontal Wireline Services on Tuesday, July 17, 2012 in Irwin, Pa. (AP Photo/Evan Vucci)
Republican presidential candidate, former Massachusetts Gov. Mitt Romney smiles as he addresses supporters at his Super Tuesday campaign rally in Boston, Tuesday, March 6, 2012. (AP Photo/Stephan Savoia)
Republican presidential candidate, former Massachusetts Gov. Mitt Romney holds One-year-old Madison Busch during a campaign rally in Loveland, Colo., Tuesday, Feb. 7, 2012. (AP Photo/Gerald Herbert)
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Romney, a former governor of Massachusetts, is expected to make a case that Trump as U.S. president would damage the country at home and abroad.
"His domestic policies would lead to recession," Romney will say. "His foreign policies would make America and the world less safe. He has neither the temperament nor the judgment to be president."
As Clinton moves closer to becoming the Democratic nominee for November's presidential election, Romney will say that any decision to make Trump the Republican counterpart would bolster Clinton's chances of succeeding President Barack Obama.
"A person so untrustworthy and dishonest as Hillary Clinton must not become president," Romney is due to say. "But a Trump nomination enables her victory."
It was only four years ago that Romney and his wife, Ann, traveled to Las Vegas to accept Trump's endorsement at a news conference where Romney praised Trump -- who had taken to questioning Obama's birthplace and eligibility -- for "an extraordinary ability to understand how our economy works and to create jobs."
Trump didn't hesitate to point that out in a Twitter post Thursday, "Why did Mitt Romney BEG me for my endorsement four years ago?" Romney passed on a 2016 campaign himself and has grown more critical of Trump as the billionaire's campaign progressed. Days before Super Tuesday, Romney suggested Trump may be delaying the release of his tax returns because they contain a "bombshell."
The irony wasn't lost on those who recalled the pressure Romney came under in 2012 to release his own tax returns, including taunting by then-Senate Majority Leader Harry Reid. More recently, after Trump failed to disavow the support of former Ku Klux Klan leader David Duke during a TV interview last weekend, Romney said the mogul had disqualified himself.
On NBC Thursday, Trump said "I disavow David Duke and the KKK and anybody else who preaches hate."
Romney's speech is the latest attempt by long-time Republicans to stop his momentum after Trump's competitors in the race for the party's nomination -- from former Florida Governor Jeb Bush to Senators Ted Cruz of Texas and Marco Rubio of Florida -- have been unable to collect more than a handful of wins at the polls. Romney will not endorse any of Trump's opponents in the speech, a Republican familiar with his plans said Wednesday.
Of 15 U.S. states that have held nominating contests to date, Trump has won 10 and has 46 percent of the delegates awarded so far.
Trump has begun to collect endorsements from high-profile elected Republicans, including New Jersey Governor Chris Christie, Maine Governor Paul LePage, and U.S. Senator Jeff Sessions of Alabama.
After Trump's Super Tuesday wins this week, a pair of super-PACs that oppose him stepped up their advertising efforts as Republicans wrestled with whether and how to stop Trump at this late stage of the nominating process -- and which alternative candidate to unite behind. The ads targeted the billionaire's Trump University as a "scam" duping "victims."
Sixty Republicans, including former World Bank President Robert Zoellick, former Homeland Security Sec. Michael Chertoff, signed an open letter opposing Trump and vowing not to support party ticket with Trump at its head.
"He is fundamentally dishonest," says the letter, coordinated by Bryan McGrath, manager of defense consultancy TheFerryBridgeGroup, and Eliot Cohen, a Defense Department and State Department adviser under Presidents George H.W. Bush and George W. Bush.