Romney has been one of the most prominent Republicans to speak out against Trump. He expressed concern about Trump when he first ran for president in 2016 and was fiercely critical of him during his four years in office.

“Donald Trump is a phony, a fraud. His promises are as worthless as a degree from Trump University,” Romney said in a major speech amid the 2016 GOP presidential primary race. “He’s playing the members of the American public for suckers. He gets a free ride to the White House and all we get is a lousy hat.”

When he first ran for Senate in 2018, Romney said that Trump is not “a role model for my grandkids.”

Notably, he was the only Senate Republican to join Democrats and vote to convict Trump during the first Senate impeachment trial in 2020. He was also among ten Senate Republicans to vote to convict Trump during the second impeachment trial in 2021 following the Jan. 6 attack on the Capitol.

Senate Minority Leader Mitch McConnell, R-Ky., in a statement on Wednesday praised Romney: The U.S. Senate is known to attract bright and proven public servants. However, we rarely get to welcome new Senators already as accomplished and well-regarded as Mitt Romney. The Senate has been fortunate to call our friend from Utah a colleague these past four and a half years, and I am sorry to learn that he will depart our ranks at the end of next year.”

Former Romney aides and advisers expressed their disappointment and a bit of surprise at his decision to retire from Congress. “He’ll get bored,” one longtime Romney aide told NBC News. “But there’s not much sense in staying in the senate another six years.”

Romney also faced pressure from within his own party. Utah House Speaker Brad Wilson said earlier this year that he was exploring the possibility of running a primary challenge against Romney. As of the end of June, Wilson had raised $2.2 million including a $1.2 million loan from the candidate himself.

In response to the announcement on Wednesday, Trump wrote a statement in all caps on Truth Social, celebrating the news.

“Fantastic news for America, the great state of Utah, & for the Republican Party. Mitt Romney, sometimes referred to as Pierre Delecto, will not be seeking a second term in the U.S. Senate, where he did not serve with distinction,” Trump wrote, referring to Romney using a pseudonym of “Pierre Delecto” to operate a Twitter account anonymously.

Trump added, “A big primary fight against him was in the offing, but now that will not be necessary. Congrats to all. Make America Great Again.”