WASHINGTON — Republican Sen. Mitch McConnell is announcing on Thursday that he won’t seek reelection next year, ending a decadeslong tenure as a power broker who championed conservative causes but ultimately ceded ground to the fierce GOP populism of President Donald Trump.

McConnell, the longest-serving Senate party leader in U.S. history, chose his 83rd birthday to share his decision not to run for another term in Kentucky and to retire when his current term ends. He informed The Associated Press of his decision before he was set to address colleagues in a speech on the Senate floor.

His announcement begins the epilogue of a storied career as a master strategist, one in which he helped forge a conservative Supreme Court and steered the Senate through tax cuts, presidential impeachment trials and fierce political fights.

“Seven times, my fellow Kentuckians have sent me to the Senate,” McConnell said in prepared remarks provided in advance to the AP. “Every day in between I’ve been humbled by the trust they’ve placed in me to do their business here. Representing our commonwealth has been the honor of a lifetime. I will not seek this honor an eighth time. My current term in the Senate will be my last.”

McConnell, first elected in 1984, intends to serve the remainder of his term ending in January 2027. The Kentuckian has dealt with a series of medical episodes in recent years, including injuries sustained from falls and times when his face briefly froze while he was speaking.

The senator plans to deliver his speech in a chamber the famously taciturn McConnell revered as a young intern long before joining its back benches as a freshman lawmaker in the mid-1980s. His dramatic announcement comes almost a year after his decision to relinquish his leadership post after the November 2024 election. South Dakota Sen. John Thune, a top McConnell deputy, replaced him as majority leader.

McConnell’s looming departure reflects the changing dynamics of the Trump-led GOP. He’s seen his power diminish on a parallel track with both his health and his relationship with Trump, who once praised him as an ally but has taken to criticizing him in caustic terms.

In Kentucky, McConnell’s departure will mark the loss of a powerful advocate and will set off a competitive GOP primary next year for what will now be an open Senate seat. Kentucky Democratic Gov. Andy Beshear, seen as a rising star in his party for winning statewide office in Republican territory, has said he has no interest in the Senate, though he is widely viewed as a contender for higher office.

McConnell, a diehard adherent to Ronald Reagan’s brand of traditional conservatism and muscular foreign policy, increasingly found himself out of step with a GOP shifting toward the fiery, often isolationist populism espoused by Trump.

McConnell still champions providing Ukraine with weapons and other aid to fend off Russia’s invasion, even as Trump ratchets up criticism of the country and its leader, Volodymyr Zelenskyy. The senator plans to make it clear Thursday that national defense remains at the forefront of his agenda.

“Thanks to Ronald Reagan’s determination, the work of strengthening American hard power was well underway when I arrived in the Senate,” McConnell said in his prepared remarks. “But since then, we’ve allowed that power to atrophy. And today, a dangerous world threatens to outpace the work of rebuilding it. So, lest any of our colleagues still doubt my intentions for the remainder of my term: I have some unfinished business to attend to.”

McConnell and Trump were partners during Trump’s first term, but the relationship was severed after McConnell blamed Trump for “disgraceful” acts in the Jan. 6, 2021, Capitol attack by his supporters. A momentary thaw in 2024 when McConnell endorsed Trump didn’t last.

Last week, Trump referred to McConnell as a “very bitter guy” after McConnell, who battled polio as a child, opposed vaccine skeptic Robert F. Kennedy Jr.’s confirmation as the nation’s top health official. McConnell referred to Trump as a “despicable human being” and a “narcissist” in a biography of the senator by The Associated Press’ deputy Washington bureau chief, Michael Tackett.

Before their falling out, Trump and McConnell pushed through a tax overhaul largely focused on reductions for businesses and higher-earning taxpayers. They joined forces to reshape the Supreme Court when Trump nominated three justices and McConnell guided them to Senate confirmation, tilting the high court to the right.

McConnell set a new precedent for hardball partisan tactics in 2016 by refusing to even give a hearing to Democratic President Barack Obama’s pick of Merrick Garland to replace the late Supreme Court Justice Antonin Scalia. Putting the brakes on the Senate’s “advise and consent” role for judicial nominees, McConnell said the vacancy should be filled by the next president so voters could have their say. Trump filled the vacancy once he took office, and McConnell later called the stonewalling of Garland’s nomination his “most consequential” achievement.

Later, when liberal Justice Ruth Bader Ginsburg died weeks before the 2020 presidential election won by Democrat Joe Biden, McConnell rushed Amy Coney Barrett’s confirmation through the Senate, waving off allegations of hypocrisy.

McConnell also guided the Senate — and Trump — through two impeachment trials that ended in acquittals.

In the second impeachment, weeks after the deadly Capitol attack by a mob hoping to overturn Trump’s 2020 reelection defeat, McConnell joined all but seven Republicans in voting to acquit. McConnell said he believed Trump couldn’t be convicted because he’d already left office, but the senator also condemned Trump as “practically and morally responsible” for the insurrection.

McConnell over the years swung back and forth from majority to minority leader, depending on which party held power. He defended President George W. Bush’s handling of the Iraq war and failed to block Obama’s health care overhaul.

McConnell, the longest-serving senator ever from Kentucky, ensured that the Bluegrass State received plenty of federal funding. Back home he was a key architect in his party’s rise to power in a state long dominated by Democrats.

He is married to Elaine Chao, and they have long been a power couple in Washington. In his prepared remarks Thursday, the senator referred to her as his “ultimate teammate and confidante.” Chao was labor secretary for Bush and transportation secretary during Trump’s first term, though she resigned after the Capitol insurrection, saying it had “deeply troubled” her.

McConnell’s parting words reflected his devotion to the Senate and his disdain for his detractors.

“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.”