WASHINGTON — Sen. Mitch McConnell, the longest-serving Senate leader in history who maintained his power in the face of dramatic convulsions in the Republican Party for almost two decades, will step down from that position in November.

McConnell, who turned 82 last week, announced his decision Wednesday in the well of the Senate, the chamber where he looked in awe from its back benches in 1985 when he arrived and where he grew increasingly comfortable in the front row seat afforded the party leaders.

“One of life’s most underappreciated talents is to know when it’s time to move on to life’s next chapter,” he said. “So I stand before you today ... to say that this will be my last term as Republican leader of the Senate.”

His decision punctuates a powerful ideological transition underway in the Republican Party, from President Ronald Reagan’s brand of traditional conservatism and strong international alliances, to the fiery, often isolationist populism of former President Donald Trump.

McConnell said he plans to serve out his Senate term, which ends in January 2027, “albeit from a different seat in the chamber.”

He spoke at times haltingly, his emotions evident, as he looked back on his career. Dozens of members of his staff lined up behind him on the back wall of the chamber, some wiping away tears, as family and friends looked down from the gallery above. Senators from both parties — most of them taken by surprise by the announcement — trickled into the chamber and exchanged hugs and handshakes.

President Joe Biden, who has had a productive working relationship with McConnell, said he was sorry to hear the news.

“I’ve trusted him and we have a great relationship,” the Democratic president said. “We fight like hell. But he has never, never, never misrepresented anything.”

Aides said McConnell’s announcement was unrelated to his health. The Kentucky senator had a concussion from a fall last year and two public episodes where his face briefly froze while he was speaking.

“As I have been thinking about when I would deliver some news to the Senate, I always imagined a moment when I had total clarity and peace about the sunset of my work,” McConnell said. “A moment when I am certain I have helped preserve the ideals I so strongly believe. It arrived today.”

The senator had been under increasing pressure from the restive and at times hostile wing of his party that has aligned firmly with Trump. The two have been estranged since December 2020, when McConnell refused to abide Trump’s lie that the election of Democrat Biden as president was the product of fraud.

But while McConnell’s critics within the GOP conference had grown louder, their numbers had not grown appreciably larger, a marker of McConnell’s strategic and tactical skill and his ability to understand the needs of his fellow Republican senators.

McConnell gave no specific reason for the timing of his decision, which he has been contemplating for months, but he cited the recent unexpected death of his wife’s youngest sister in a car crash as a moment that prompted introspection. “The end of my contributions are closer than I’d prefer,” McConnell said.

But his remarks were also light at times as he talked about the arc of his Senate career.

He noted that when he arrived in the Senate, “I was just happy if anybody remembered my name.” During his campaign in 1984, when Reagan was visiting Kentucky, the president called him “Mitch O’Donnell.”

McConnell endorsed Reagan’s view of America’s role in the world, and the senator has persisted in the face of opposition, including from Trump, that Congress should include a foreign assistance package that includes $60 billion for Ukraine.

After his speech, Sen. Susan Collins, R-Maine, congratulated him in brief remarks, saying that she admired him “for stepping forward when it wasn’t popular to do the right thing for our country and our world.”

Trump has pulled the party hard to the ideological right, questioning longtime military alliances such as NATO, international trade agreements and pushing for a severe crackdown on immigration, all the while clinging to the falsehood that the election was stolen from him in 2020.

McConnell and Trump had worked together during Trump’s time in the White House, remaking the Supreme Court and the federal judiciary in a far more conservative image, and on tax legislation. But there was also friction from the start, with Trump frequently sniping at the senator.

Their relationship has essentially been over since Trump refused to accept the results of the Electoral College. But the rupture deepened dramatically after the Jan. 6, 2021, attack on the Capitol. McConnell assigned blame and responsibility to Trump and said that he should be held to account through the criminal justice system for his actions.

McConnell’s path to power was hardly linear, but from the day he walked onto the Senate floor in 1985 and took his seat as the most junior Republican senator, he set his sights on being the party leader. What set him apart was that so many other Senate leaders wanted to run for president. McConnell wanted to run the Senate. He lost races for lower party positions before steadily ascending, and finally became party leader in 2006 and has won nine straight elections.

He most recently beat back a challenge led by Sen. Rick Scott of Florida last November.