President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE endorsed Mitt Romney Willard (Mitt) Mitt RomneyMomentum growing among Republicans for Supreme Court vote before Election Day Toobin: McConnell engaging in 'greatest act of hypocrisy in American political history' with Ginsburg replacement vote The Memo: Court battle explodes across tense election landscape MORE in the Senate race to replace retiring Sen. Orrin Hatch Orrin Grant HatchBottom line Bottom line Senate GOP divided over whether they'd fill Supreme Court vacancy MORE (R-Utah) on Monday night, despite reportedly urging Hatch to run for reelection to block Romney out.

“He will make a great Senator and worthy successor to @OrrinHatch, and has my full support and endorsement!” Trump tweeted Monday.

.@MittRomney has announced he is running for the Senate from the wonderful State of Utah. He will make a great Senator and worthy successor to @OrrinHatch, and has my full support and endorsement! — Donald J. Trump (@realDonaldTrump) February 19, 2018

Romney thanked the president in a tweet of his own.

Thank you Mr. President for the support. I hope that over the course of the campaign I also earn the support and endorsement of the people of Utah. — Mitt Romney (@MittRomney) February 19, 2018

Trump and Romney have had a complicated relationship. The 2012 GOP presidential nominee was a frequent critic during Trump’s own presidential campaign.

In March of 2016, Romney said he would have refused Trump's endorsement four years earlier if he had known the real estate mogul's true colors.

If Trump had said 4 years ago the things he says today about the KKK, Muslims, Mexicans, disabled, I would NOT have accepted his endorsement — Mitt Romney (@MittRomney) March 3, 2016

Still, Romney met with Trump over the possibility of becoming secretary of State after Trump won the election.

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Romney has taken up his criticism of Trump again after being passed over for the job, hitting the president over his comments on the white supremacist rally in Charlottesville, Va., and his endorsement of Republican Roy Moore Roy Stewart MooreVulnerable Senate Democrat urges unity: 'Not about what side of the aisle we're on' Sessions hits back at Trump days ahead of Alabama Senate runoff Judge allows Roy Moore lawsuit over Sacha Baron Cohen prank to proceed MORE in the Alabama Senate special election.

He also slammed Trump for referring to Haiti, El Salvador and some African nations as “shithole countries.”

Reports emerged late last year that Trump was pressing Hatch to run for reelection to prevent what many see as a cakewalk bid for Romney.

“We hope you will continue to serve your state and the country in the Senate for a very long time to come,” Trump told Hatch during a speech in Salt Lake City.

However, former Romney campaign staffers told The Hill that they believed the former governor wouldn’t run as a protest candidate, but wouldn’t be afraid to speak out against Trump.

“The governor understands that the president is the leader of the political party, the Republican Party,” said Ryan Williams, a Republican strategist who worked on Romney’s 2012 campaign. “The president shapes the party. When the governor was the nominee in 2012, he was the de facto leader of the party, but now President Trump is. But that doesn’t mean that will affect his views.”

Romney made his long-suspected Senate bid official on Friday, saying now "is the right time to serve" Utah and its people.

"I have decided to run for United States Senate because I believe I can help bring Utah's values and Utah's lessons to Washington. Utah is a better model for Washington than Washington is for Utah," he said in a video announcing his campaign.