Senator Mitt Romney of Utah, the 2012 Republican nominee for president who made a historic break with his party when he voted to remove former President Donald J. Trump from office, announced on Wednesday that he would not seek re-election in 2024, saying he wanted to make way for a “new generation of leaders.”

He strongly suggested that Mr. Trump, 77, and President Biden, 80, should follow his lead and bow out to pave the way for younger candidates, arguing that neither was effectively leading his party to confront the “critical challenges” the nation faces.

“At the end of another term, I’d be in my mid-80s. Frankly, it’s time for a new generation of leaders,” Mr. Romney, 76, said in a video statement. “They’re the ones that need to make the decisions that will shape the world they will be living in.”

The announcement was in some ways the culmination of a long divergence between Mr. Romney, a genteel and wealthy former governor and traditional conservative, and the Republican Party, which has veered sharply to the right and embraced a coarser brand of partisanship in recent years.

Elected to the Senate in 2018, Mr. Romney has occupied a lonely space in a Capitol where a majority of Republicans remain loyal to Mr. Trump — or at least refuse to break with him. Mr. Romney has joined an array of bipartisan “gangs” seeking to take on major policy issues — including infrastructure, gun safety and overhauling the Electoral Count Act — but rarely sought to lead those efforts.

In the video, Mr. Romney said that neither Mr. Biden nor Mr. Trump, the current front-runner for the Republican nomination, was addressing the nation’s most critical challenges, including climate change, authoritarian threats from Russia and China and mounting debt.

“Both men refuse to address entitlements even though they represent two-thirds of federal spending,” he said. “Donald Trump calls global warming a hoax, and President Biden offers feel-good solutions that will make no difference to the global climate. On China, President Biden underinvests in the military, and President Trump underinvests in our alliances.”

“The next generation of leaders must take America to the next stage of global leadership,” he added.

The statement came amid renewed scrutiny of the age of Mr. Biden and other prominent elected officials including Senator Mitch McConnell of Kentucky, 81, the longtime Republican leader whose recent health issues have raised questions about whether he is fit to continue in his post. Editors’ Picks MTV Video Music Awards: 5 Memorable Moments Why Are So Many Millennials Going to Mongolia? My Disabled Colleague Is Struggling at Work. Am I Responsible for Her Care? SKIP ADVERTISEMENT

Mr. Romney, who describes his career in politics as a moral mission driven by his Mormon faith, has in recent years been marginalized in a party that has shifted to the right under the sway of Mr. Trump. In the Senate, he never emerged as a leader of any faction or committee, even as he was regarded as a reliable, common-sense vote.

He hinted that he might still have some role to play in the nation’s political discourse, saying, “I’m not retiring from the fight.” He said he planned to finish out his term, which ends in January 2025.

Utah is a solidly Republican state, so Mr. Romney’s departure is highly unlikely to affect the balance of power on Capitol Hill. He had recently told people that he planned to make a decision about seeking re-election by the end of the year and that he was weighing whether he could still play any productive role in Congress. Mr. Romney was also cognizant that he would face a tough primary fight if he decided to run again.

His decision to abandon a career in the Senate followed similar decisions from many moderate House Republicans last year. In the 2022 midterm elections, four House Republicans who voted to impeach Mr. Trump declined to run for re-election.

Mr. Romney had also begun to stir speculation that he was ready to move on from the Senate when he agreed to participate in a biography set to be published next month by Scribner, titled “Romney: A Reckoning,” by McKay Coppins, a staff writer at The Atlantic. In the book, Mr. Romney is said to quote his colleagues by name in discussing how Republican lawmakers really view and talk about Mr. Trump in private when the former president is not present.

Mr. Coppins is said to have conducted hours of interviews with Mr. Romney for the book, and was given access to the senator’s emails and his diary. The book’s impending release already has his colleagues concerned about their private thoughts and conversations regarding the party’s vengeful presidential front-runner being aired publicly.

Mr. Romney has also appeared increasingly concerned about the likelihood that Mr. Trump would emerge as his party’s nominee.

In a recent opinion piece in The Wall Street Journal, Mr. Romney implored donors and Republican candidates to unite around an alternative to Mr. Trump, for fear of delivering him the party’s nomination, writing that “donors who are backing someone with a slim chance of winning should seek a commitment from the candidate to drop out and endorse the person with the best chance of defeating Mr. Trump by Feb. 26.”