In his new role as Donald Trump's chief adversary, former Republican presidential nominee Mitt Romney has turned his alarm-fueled message into a robocall on behalf of struggling GOP contender Sen. Marco Rubio, NBC News has confirmed.
Romney's warning is coming to a phone near you in four states voting Tuesday: Michigan, Idaho, Hawaii and Mississippi.
In the call, which was first reported by the New York Times, Romney notifies listeners that he is calling for Rubio and asks them to choose "a candidate who can defeat Hillary Clinton and who can make us proud."
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Marco Rubio on the campaign trail
Mitt Romney records anti-Trump robocall for Marco Rubio
IN FLIGHT - FEBRUARY 10: Republican presidential candidate Sen. Marco Rubio (R-FL) talks with reporters on his charter flight from Manchester-Boston Regional Airport February 10, 2016 en route to Spartanburg, South Carolina. Rubio placed fifth in the New Hampshire primary, behind fellow GOP candidates Jeb Bush, John Kasich, Sen. Ted Cruz (R-TX) and Donald Trump, who swept away the competition with 35-percent of the vote. (Photo by Chip Somodevilla/Getty Images)
MT PLEASANT, SC - JANUARY 13: Republican presidential candidate Sen. Marco Rubio (R-FL) greets guests during a campaign rally at the Water Dog Grill on January 13, 2016 in Mount Pleasant, South Carolina. Tomorrow Rubio will join other candidates seeking the Republican nomination for president for a debate at the North Charleston Coliseum and Performing Arts Center in North Charleston, S.C.. (Photo by Scott Olson/Getty Images)
Senator Marco Rubio, a Republican from Florida and 2016 presidential candidate, waits to speak during a town hall meeting at the Maytag Innovation Center in Newton, Iowa, U.S., on Wednesday, Dec. 30, 2015. Rubio found himself under siege on two fronts Tuesday over his attendance record in the U.S. Senate as the Florida lawmaker embarked on a swing through snowy Iowa. Photographer: Scott Morgan/Bloomberg via Getty Images
MIAMI BEACH, FL - NOVEMBER 15: Republican presidential candidate Sen. Marco Rubio (R-FL) greets people before speaking during a community rally for 'Never Again' which was bringing attention to what the organizers say is a rise in worldwide anti-Semitism and the campaign against Israel's right to exist on November 15, 2015 in Miami Beach, Florida. Rubio continues to campaign for the Republican party's nomination. (Photo by Joe Raedle/Getty Images)
CEDAR RAPIDS, IA - NOVEMBER 21: Republican presidential candidate Sen. Marco Rubio (R-FL) speaks to guests during a town hall meeting on November 21, 2015 in Cedar Rapids, Iowa. Yesterday Rubio participated in the Presidential Family Forum in Des Moines with six of his Republican rivals for the nomination. Rubio has several campaign stops scheduled in the state today. (Photo by Scott Olson/Getty Images)
Republican presidential candidate Marco Rubio speaks at the Wall Street Journal CEO Council meetings in Washington, DC, November 16, 2015. AFP PHOTO / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images)
ORLANDO, FL - NOVEMBER 12: Sen. Marco Rubio (R-FL) speaks at the Sunshine Summit opening dinner at Disney's Contemporary Resort on November 12, 2015 in Orlando, Florida.The dinner is the kick-off of a three-day event that will draw thousands of Republicans, mostly to hear live speeches from all the GOP presidential candidates on Friday and Saturday. (Photo by Tom Benitez - Pool/Getty Images)
ORLANDO, FL - NOVEMBER 13: Republican presidential candidate Sen. Marco Rubio (R-FL) speaks during the Sunshine Summit conference being held at the Rosen Shingle Creek on November 13, 2015 in Orlando, Florida. The summit brought Republican presidential candidates in front of the Republican voters. (Photo by Joe Raedle/Getty Images)
Senator Marco Rubio, a Republican from Florida and 2016 presidential candidate, speaks during a campaign rally at the Country Springs Hotel in Pewaukee, Wisconsin, U.S., on Monday, Nov. 9, 2015. Rubio will appear at Tuesday's Republican presidential debate in Milwaukee. Photographer: Daniel Acker/Bloomberg via Getty Images
CONCORD, NH - NOVEMBER 05: Republican Presidential candidate Marco Rubio (R-FL) files paperwork for the New Hampshire primary at the State House on November 5, 2015 in Concord, New Hampshire. Each candidate must file paperwork to be on the New Hampshire primary ballot, which will be held February 9, 2016. (Photo by Scott Eisen/Getty Images)
Republican Presidential hopeful Marco Rubio leaves the New Hampshire State House after filing for the state ballot November 5, 2015 in Concord, New Hampshire. AFP PHOTO/DON EMMERT (Photo credit should read DON EMMERT/AFP/Getty Images)
MANCHESTER, NH - NOVEMBER 4: Republican Presidential candidate Marco Rubio (R-FL) participates in a round table discussion at Saint Anselm College in Manchester, New Hampshire. Rubio is looking for a bump in the polls following a strong outing in the last debate. (Photo by Darren McCollester/Getty Images)
GREENVILLE, SC - SEPTEMBER 18: Sen. Marco Rubio (R-FL) speaks to voters at the Heritage Action Presidential Candidate Forum September 18, 2015 in Greenville, South Carolina. Eleven republican candidates each had twenty five minutes to talk to voters Friday at the Bons Secours Wellness arena in the upstate of South Carolina. (Photo by Sean Rayford/Getty Images)
NEW YORK, NY - OCTOBER 06: Republican presidential candidate, Sen. Marco Rubio (R-FL) speaks at Civic Hall about the 'sharing economy' on October 6, 2015 in New York City. Rubio, who has been experiencing a slight uptick in the polls after strong debate performances, has a second book out in paperback this Tuesday called American Dream: Restoring Economic Opportunity for Everyone. (Photo by Spencer Platt/Getty Images)
UNITED STATES - AUGUST 26: GOP Presidential candidate Sen. Marco Rubio, R-Fla., holds a town hall meeting in Londonderry, N.H., on Wednesday, Aug. 26, 2015. (Photo By Bill Clark/CQ Roll Call)
DES MOINES, IA - AUGUST 18: Republican presidential candidate and U.S. Sen. Marco Rubio (L) (R-FL) mans the grill with U.S. Rep. David Young (R) (R-IA) at the Iowa Pork Producers Pork Tent during the Iowa State Fair on August 18, 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 Justin Sullivan/Getty Images)
UNITED STATES - August 17: Republican presidential candidate Sen. Marco Rubio, R-Fla., greets supporters during a Family Night event at Dean Park in Ankeny, Iowa, Monday, August 17, 2015. (Photo By Al Drago/CQ Roll Call)
NEW YORK, NY - AUGUST 14: Marco Rubio gives speach for the Foreign Policy Initiative at 3 West Club on August 14, 2015 in New York City. (Photo by Steve Sands/WireImage)
CLEVELAND, OH - AUGUST 06: Republican presidential candidate Sen. Marco Rubio (R-FL) participates 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)
CLEVELAND, OH - AUGUST 05: Republican presidential candidate Sen. Marco Rubio (R-FL) greets guests gathered for a campaign event at Town Hall on August 5, 2015 in Cleveland, Ohio. Rubio is in Cleveland to participate in the Fox News GOP presidential candidate debate scheduled to take place tomorrow evening. The top ten polling Republican candidates were chosen to participate in the debate. (Photo by Scott Olson/Getty Images)
Senator Marco Rubio, a Republican from Florida and 2016 Republican presidential candidate, smiles while speaking during a rally at Town Hall restaurant ahead of the Fox News Republican Presidential Primary Debate in Cleveland, Ohio, U.S., on Wednesday, Aug. 5, 2015. Rubio will appear on stage with 9 other presidential candidates for the first Republican presidential debate tomorrow evening while former Texas Governor Rick Perry, former Pennsylvania Senator Rick Santorum, and Louisiana Governor Bobby Jindal didn't make the cut. Photographer: Andrew Harrer/Bloomberg via Getty Images
Senator Marco Rubio, a Republican from Florida and 2016 presidential candidate, speaks during the Faith and Freedom Coalition's 'Road to Majority' legislative luncheon in Washington, D.C., U.S., on Thursday, June 18, 2015. The annual Faith & Freedom Coalition Policy Conference gives top-tier presidential contenders as well as long shots a chance to compete for the large evangelical Christian base in the crowded Republican primary contest. Photographer: Andrew Harrer/Bloomberg via Getty Images
BOONE, IA - JUNE 06: Republican presidential hopeful U.S. Sen. Marco Rubio (R-FL) waits his turn to speak at a Roast and Ride event hosted by freshman Senator Joni Ernst (R-IA) on June 6, 2015 in Boone, Iowa. Ernst is hoping the event, which featured a motorcycle tour, a pig roast, and speeches from several 2016 presidential hopefuls, becomes an Iowa Republican tradition. (Photo by Scott Olson/Getty Images)
GREENVILLE, SC - MAY 09: Republican Presidential candidate U.S. Sen. Marco Rubio (R-FL) greets supporters at the Freedom Summit on May 9, 2015 in Greenville, South Carolina. Rubio joined eleven other potential candidates in addressing the event hosted by conservative group Citizens United. (Photo by Richard Ellis/Getty Images)
NASHUA, NH - APRIL 17: Republican presidential candidate Sen. Marco Rubio (R-FL) speaks at the First in the Nation Republican Leadership Summit April 17, 2015 in Nashua, New Hampshire. The Summit brought together local and national Republicans and was attended by all the Republicans candidates as well as those eyeing a run for the nomination. (Photo by Darren McCollester/Getty Images)
MIAMI, FL - APRIL 13: Republican Senator Marco Rubio of Florida makes a formal announcement that he is entering the 2016 Presidential race at the Freedom Tower on April 13, 2015 in Miami, Florida. (Photos by Charles Ommanney for the Washington Post)
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"If we Republicans were to choose Donald Trump as our nominee, I believe that the prospects for a safe and prosperous future would be greatly diminished — and I'm convinced Donald Trump would lose to Hillary Clinton," Romney says.
A source close to Romney told NBC News Monday that the former Massachusetts governor "has offered and is glad to help Senator Marco Rubio, Senator Ted Cruz, and Governor John Kasich in any way he can. He's been clear that he believes that Donald Trump is not the best person to represent the Republican Party and will do what he can to support a strong nominee who holds conservative values to win back the White House."
Romney, who has not endorsed a candidate, began his anti-Trump crusade last week, announcing during a speech in Utah that the GOP front-runner is a phony, fraud and a misogynist.
To which Trump, recalling Romney's solicitation of his endorsement in 2012, responded: "I could have said, 'Mitt drop to your knees,' and he would have dropped to his knees."