Senator Mitch McConnell, the Kentucky Republican and longest-serving Senate leader who played a pivotal role in obstructing major Democratic agenda items and stacking the federal courts with conservatives, said on Thursday that he would not seek another term in 2026.
In a speech on the Senate floor that fell on his 83rd birthday, Mr. McConnell made official what had been widely expected since he announced last year that he would step down as the Republican leader. Representing Kentucky was “the honor of a lifetime,” he said, but “I will not seek this honor an eighth time. My current term in the Senate will be my last.”
When he stepped down as leader, Mr. McConnell had said he was committed to finishing out his seventh term in Congress. He had not announced his political plans, but it had become clear that he was nearing the end of his career. Mr. McConnell has dealt with a series of health issues over the past year, including back-to-back falls recently that left him temporarily using a wheelchair to navigate the Capitol.
Mr. McConnell established himself as a master tactician in the Senate during 18 years as minority and majority leader, making shrewd use of the chamber’s rules to thwart his opponents and empower his allies, including President Trump. He blocked President Barack Obama from filling a Supreme Court seat toward the end of his tenure and then led a Republican effort to install deeply conservative jurists on the bench under the first Trump administration, culminating in the confirmation of three Supreme Court justices.
But he has a deeply fraught relationship with Mr. Trump, despite having played a key role in enacting the president’s agenda and allowing him to return to power. In recent weeks, he has found himself increasingly isolated within his own party, particularly on the issues of national security and safeguarding democracy.
In his speech on Thursday, Mr. McConnell warned that “a dangerous world threatens to outpace the work of rebuilding it.”
“I assure our colleagues that I will depart with great hope for the endurance of the Senate as an institution,” he said. “There are many reasons for pessimism, but the strength of the Senate is not one of them.”
These days, Mr. McConnell has found himself on an island in the Republican conference that he once led. In recent weeks, he has voted against three of Mr. Trump’s cabinet nominees — Pete Hegseth as defense secretary, Tulsi Gabbard as director of national intelligence and Robert F. Kennedy Jr. as health secretary — cementing his unlikely role as the main G.O.P. holdout to the president’s agenda at the beginning of his second term. Some of his colleagues said they expected him to vote late Thursday against the confirmation of Kash Patel as F.B.I. director as well.
Mr. McConnell did not mention Mr. Trump in his speech on Thursday, but he hinted that he intended to continue playing the more liberated role he has chosen for himself during his final months in Congress.
“The Senate is still equipped for work of great consequence,” he said. “And to the disappointment of my critics, I’m still here on the job.”
Some new Republican senators simply shrugged at the longtime party leader’s decision to retire and at the stance he has taken in opposition to Mr. Trump’s most scandal-tainted nominees, writing off Mr. McConnell as irrelevant.
“The Republican majority is unified in backing President Trump,” said Senator Jim Banks, a first-term Republican from Indiana. He said Senator John Thune, Republican of South Dakota and Mr. McConnell’s successor as majority leader, is “keeping us unified.”
“McConnell is the anomaly,” he continued. “It’s a nonfactor.”
Senator Ted Cruz, Republican of Texas, also played down the significance. “It’s not complicated: There’s no love lost between Mitch and President Trump, and no one is hiding that,” he said.
Mr. Trump recently cast doubt on whether Mr. McConnell, who often talks about surviving childhood polio, actually had the disease after the senator voted against the confirmation of Mr. Kennedy, a vaccine skeptic.
“I have no idea if he had polio,” Mr. Trump said. “He votes against almost everything. Now he’s a very bitter guy, and we have a very strong party, and he’s almost — not even really a very powerful member. Let’s say he’s lost his power, and it’s affected his vote.”
Unfettered by the constraints of leadership, Mr. McConnell has been emboldened to break ranks with his party and cast votes with his own place in history, rather than the interests of his conference, in mind. But because a lone Republican “no” vote is not enough to tank any of Mr. Trump’s nominees, Mr. McConnell’s are in effect messaging votes that serve only to register his opposition.
And the protest votes fly in the face of the rest of his political legacy.
Mr. McConnell may be remembered above all else for voting to acquit Mr. Trump in his impeachment trial after the Jan. 6, 2021, attack on the Capitol. That critical move closed off the only certain avenue for blocking Mr. Trump’s return to power: a Senate vote to disqualify him from running again.
“He was uniquely suited to have taken us down a different path and chose not to,” Sarah Longwell, an anti-Trump Republican strategist, said on “The Bulwark Podcast.” Ms. Longwell said she was often careful not to castigate Republicans who took a stand against Mr. Trump’s worst instincts, even if those efforts were belated.
“It’s never too late to do the right thing,” she said. But she said she viewed Mr. McConnell as a special case.
“Mitch McConnell made a series of bad choices for power that he knew were wrong for the Republican Party, that were wrong for the country,” she said. “I do think he’s going to live out the rest of his life looking at what he wrought and regretting it deeply.”
Some Democrats were not as harsh and said Mr. McConnell had pivoted in important ways after the attack on the Capitol.
“He operated in a fundamentally different manner as the leader after Jan. 6,” said Senator Christopher S. Murphy, Democrat of Connecticut. He noted that Mr. McConnell had pushed through major bipartisan bills like gun safety legislation, a $1 trillion infrastructure package and legislation to overhaul how Congress certifies presidential elections to avoid a repeat of the 2021 debacle.
“We did those things because Mitch McConnell decided that he wanted to show that democracy could work,” Mr. Murphy said. “Do I wish that he had gotten rid of this guy by supporting impeachment? Of course, but I think we should give him credit for making a decision after Jan. 6 to not fold in with Trump in the way that most did.”
Mr. McConnell himself has been uncomfortable with being viewed as a member of any sort of “resistance” to the G.O.P., even though it is now the party of Mr. Trump. He has been more eager to highlight the 16 Trump nominees whom he voted to confirm, rather than to dwell on the handful he opposed.
“I expect to support most of what this administration is trying to accomplish,” he said on “60 Minutes” this month.
In his post-election news conference last November, Mr. McConnell told reporters that “I’m going to do everything I can to help the new administration be successful.”