Mr. Romney, a critic of the president, officially confirmed that he would run to fill the seat of Senator Orrin G. Hatch, who is retiring.
In a long-expected announcement, Mitt Romney said on Friday that he would run for the United States Senate, pledging to bring Utah’s priorities of balanced budgets, strong economy and welcoming borders to Washington.
Mr. Romney, a former governor of Massachusetts and the 2012 Republican nominee for president, made the announcement in a two-minute video posted to Twitter and Facebook. He cited his experience as the chief executive of the organizing committee for the 2002 Winter Olympics in Salt Lake City and compared Utah, where his mother was born, favorably to the nation’s capital.
“Utah has a lot to teach the politicians in Washington,” he said, contrasting the state’s balanced budgets and positive export-to-import-ratio to Washington’s.
And, in a pointed gesture toward the restrictive immigration policies of President Trump, he noted that “Utah welcomes legal immigrants from around the world,” while “Washington sends immigrants a message of exclusion.”
Last month, The New York Times reported that Mr. Romney had said privately that he was planning a Senate campaign. “I’m running,” he texted a close friend, Kem Gardner.
He would replace Senator Orrin G. Hatch, who announced in early January that he would retire at the end of the year.
Mr. Romney is popular in Utah, despite not having been born in the state and having spent much of his life elsewhere. (He and his wife, Ann, once kept a vacation home in Park City and in 2013, built a mansion in Holladay, a suburb of Salt Lake City.) He is one of the most prominent Mormons in American public life, a political advantage in a state where the population is about 60 percent Mormon and many civic and business leaders attend the Church of Jesus Christ of Latter-day Saints.
Speaker Paul D. Ryan, Mr. Romney’s running mate in 2012, endorsed him in a statement on Friday, saying, “Our party and our country are always better off when Mitt is engaged.”
Mr. Trump had pleaded with Senator Hatch, the longest-serving Senate Republican, to run for an eighth term, and Mr. Romney’s video underscored the potential for continuing clashes between himself and the president. The clip took several barely veiled shots at Mr. Trump’s policies and manner, as well as the ever-coarsening state of political discourse on Washington’s Capitol Hill.
“On Utah’s Capitol Hill,” he said, “people treat one another with respect.”
During the 2016 campaign, Mr. Romney decried Mr. Trump as a “fraud” and a “phony,” and said that he was “playing the American public for suckers.” But after the election, he embarked on a more cordial relationship with the president, as his name was one of several in the discussion of possible nominees for Secretary of State. After the violent events in Charlottesville, Va., last summer, he again changed his approach, imploring the president to apologize for saying that there had been blame on both sides.
“What he communicated caused racists to rejoice, minorities to weep, and the vast heart of America to mourn,” he said in a Facebook post in August.
The two have talked more recently, but Mr. Trump has not offered his support for Mr. Romney’s campaign. The White House did not immediately respond to a request for comment on the announcement.
Mr. Romney is well-known to many Americans, having run for president twice, once in 2008 and again in 2012, when he faced Barack Obama as the Republican Party’s nominee. After losing, he remained relatively visible, first with the debut of the 2014 documentary “Mitt,” an intimate look at his campaigns that made Mr. Romney seem less awkward and ill-at-ease than he often had while campaigning, and then with his comments about Mr. Trump in 2016.
