Senator Orrin G. Hatch of Utah, the longest-serving Senate Republican, announced Tuesday that he will retire at the end of the year, rebuffing personal pleas from President Trump to seek an eighth term and paving the way for Mitt Romney, a critic of Mr. Trump’s, to run for the seat.
Mr. Hatch’s decision marks another political setback for Mr. Trump, who lost a Senate seat in Alabama after his preferred candidates were rejected. He also faces an exodus of Republicans from both chambers of Congress and has been warned of a political typhoon in November.
Mr. Romney’s potential ascent is particularly alarming to the White House because the former presidential candidate has an extensive political network and could use the Senate seat as a platform to again seek the nomination. Even if he were not to run again for president, a Senator Romney could prove a pivotal swing vote, impervious to the entreaties of a president he has scorned and able to rally other Trump skeptics in the chamber.
“When there are things he agrees with him on, he’ll be a big supporter,” Spencer Zwick, Mr. Romney’s longtime fund-raiser, said of Mr. Romney and Mr. Trump. “And when there are things he disagrees with, he’ll voice that.”
Mr. Hatch, 83, made his decision public on Tuesday afternoon via a video announcement.
“When the president visited Utah last month, he said I was a fighter. I’ve always been a fighter. I was an amateur boxer in my youth, and I’ve brought that fighting spirit with me to Washington,” he said. “But every good fighter knows when to hang up the gloves. And for me, that time is soon approaching.”
Mr. Hatch was under heavy pressure from Mr. Trump to seek re-election and block Mr. Romney. Last month, Mr. Trump flew with Mr. Hatch, who has emerged as one of the president’s most avid loyalists in the Senate, on Air Force One to Utah for a day of events that was aimed entirely at lobbying the senator to run again. At the request of the senator, the president announced that he was vastly shrinking two of Utah’s sprawling national monuments, reversing decisions made by Presidents Barack Obama and Bill Clinton.
In a statement he posted on Facebook on Tuesday, Mr. Romney made no mention of his intentions; he only saluted Mr. Hatch.
“As Chairman of the Senate Finance and Judiciary Committees and as the longest-serving Republican Senator in U.S. history, Senator Hatch has represented the interests of Utah with distinction and honor,” he said.
By Tuesday evening, Mr. Romney had updated his Twitter profile to change his location to Holladay, Utah, from Massachusetts.
Mr. Romney intends to make his intentions known in a matter of weeks, according to an adviser who spoke on the condition of anonymity. His senior campaign team will include Mr. Zwick; Matt Waldrip, who had been running Mr. Romney’s annual policy retreats; and his former chief of staff, Beth Myers.
Mr. Zwick did not confirm Mr. Romney would enter the race, but said that “of all the people who can run, Mitt will represent and honor the legacy of Senator Hatch more than anybody.”
Mr. Hatch, who briefly ran for president in 2000, amassed a distinguished record over his four decades in Washington and became a fixture in a Senate once noted for its bipartisanship. While considered an institution in his home state, Mr. Hatch was facing harsh poll numbers in Utah, where 75 percent of voters indicated in a survey last fall that they did not want him to run again.
Mr. Hatch’s decision comes just weeks after Mr. Trump signed the tax overhaul into law, a measure that the senator helped shepherd as chairman of the Finance Committee. The bill represented something of a capstone to Mr. Hatch’s career, and Senator Mitch McConnell, Republican of Kentucky and the majority leader, even deemed it as such last month in what was seen as a subtle effort to usher his colleague to the exit.
Mr. Romney was unaware of Mr. Hatch’s decision and of late had been operating under the assumption that the senator would run again, not even bringing up the possibility of a campaign while skiing Monday with friends in Utah.
That is in part because Mr. Hatch had privately told Mr. Romney he was not sure he was ready to leave a seat he has held since 1977, and White House officials did all they could to nudge him into another campaign.
The president has had Mr. Romney on his mind. Over golf last year, Mr. Trump asked Senator Lindsey Graham, Republican of South Carolina, what he thought of the former Republican nominee. (Mr. Graham said he praised Mr. Romney and predicted he would be a solid senator.)
As the president prodded Mr. Hatch to stay, voices in his home state were urging him to go. On Christmas Day, The Salt Lake Tribune named the senator “Utahn of the Year,” but not for flattering reasons.
“It would be good for Utah if Hatch, having finally caught the Great White Whale of tax reform, were to call it a career. If he doesn’t, the voters should end it for him,” the editorial concluded.
In announcing his retirement, Mr. Hatch joined an exodus of Republican heavyweights in what promises to be a difficult election season. Also on Tuesday, Representative Bill Shuster of Pennsylvania, the chairman of the House Transportation Committee, disclosed that he planned to retire at the year’s end. Mr. Shuster, 56, was facing a possible primary election challenge from the right and said his decision would allow him to focus exclusively on trying to steer major infrastructure legislation, a long-sought bipartisan priority, into law.
Other retiring House chairmen include Robert W. Goodlatte of Virginia, of the Judiciary Committee; Jeb Hensarling of Texas, of the Financial Services Committee; and Lamar Smith of Texas of the Committee on Science, Space and Technology.
In all, 29 House Republicans have resigned, announced they will retire or run for another office, compared to 16 Democrats. Mr. Hatch joined Senators Bob Corker of Tennessee and Jeff Flake of Arizona, both Republicans, in announcing an end to his Senate career.
Mr. Hatch defeated a Democrat 42 years ago this November, arguing that the incumbent had stayed in Washington too long, and became one of the country’s most prominent senators for a generation. While he usually voted a conservative line, he also developed close relationships with his Democratic colleagues, most famously with Senator Edward M. Kennedy of Massachusetts. In the 1990s, the two collaborated on what became a landmark health care plan, the State Children’s Health Insurance Program, and Mr. Hatch even wrote a love song for Mr. Kennedy and his wife, Victoria, called “Souls Along The Way.”
“Orrin’s long list of accomplishments means he will depart as one of the most productive members ever to serve in this body,” Mr. McConnell said in a statement.