Senator McConnell says he will not run for an eighth term in 2026, bringing his historic political career to a close and freeing him up even further to be a thorn in the side of President Trump from his perch in the Senate. Mr. McConnell’s decision to step aside could set up a nasty fight among Kentucky Republicans for the Senate nomination. 

Mr. McConnell, at the age of 80, will leave the Senate having had the longest tenure as a chamber party leader in history — 18 years in total. He says now that he has done enough and is ready to leave. 

“Seven times, my fellow Kentuckians have sent me to the Senate,” Mr. McConnell said during a speech on the Senate floor Thursday. “Every day in between, I’ve been humbled by the trust they’ve placed in me to do their business right 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.” 

Several senators — Republicans and Democrats — were seated at their desks to listen to Mr. McConnell announce his retirement. Those who stood to applaud at the end included Democrats such as Senator Schumer, Senator Durbin, Senator Klobuchar, and Senator Booker. 

The Kentucky senator will be remembered primarily for the successes he handed Mr. Trump during his first term, including the 2017 tax bill, three justices on the Supreme Court, a new trade deal with Canada and Mexico, and other accomplishments. 

For the next two years, however, Mr. McConnell could prove himself to be a much less trustworthy ally. He has has so far voted against three of Mr. Trump’s Cabinet nominees, and he could vote against even more. Though those efforts to defeat the nominees were unsuccessful, it has clearly gotten under Mr. Trump’s skin. 

Following the confirmation of Robert F. Kennedy Jr. as health and human services secretary, the president attacked Mr. McConnell from behind the Resolute Desk because he voted against the nominee on the Senate floor. In justifying his no vote, Mr. McConnell — a childhood polio survivor — said “a record of trafficking in dangerous conspiracy theories and eroding trust in public health institutions does not entitle Mr. Kennedy to lead these important efforts.”

Mr. Trump was informed of the comments by reporters, and he did not hold back. “He shouldn’t have been leader. He knows that,” the president said. “He votes against almost everything now. He’s a, you know, a very bitter guy. We have a very strong party, and he’s almost not even a very powerful member … He’s lost his power and it’s affected his vote.”

Mr. Trump went so far as to seemingly question Mr. McConnell’s polio diagnosis. “I don’t know anything about, ‘he had polio.’ I have no idea if he had polio,” Mr. Trump said. 

Mr. McConnell has also spoken out forcefully against Mr. Trump’s tariff threats, and has maintained his support for Ukraine despite his party leader’s threats to cut off the country. 

Despite their long-running criticisms of one another, Mr. McConnell will most likely be remembered for his construction of the supermajority on the Supreme Court during Mr. Trump’s first term, which could very well last for decades. With the president’s first three nominees in their early fifties, and the increasing likelihood that Justices Clarence Thomas and Samuel Alito — both of whom are in their seventies — could step aside, it is not impossible that Mr. Trump could become the first president in decades to appoint a majority of the court. 

During a recent interview with “60 Minutes,” Mr. McConnell’s biographer stated that by holding Justice Antonin Scalia’s seat open for Mr. Trump to fill in 2017, and then moving forward with the confirmation of Justice Amy Coney Barrett in late 2020, Mr. McConnell helped orchestrate not only a conservative court, but Mr. Trump’s eventual return to the White House following his 2020 defeat. 

“McConnell thought that the criminal and civil justice system would be there to hold Trump to account for his actions,” biographer Michael Tackett said, referring to the then-Senate majority leader’s desire to see Mr. Trump held criminally liable for his actions on January 6, 2021. Those actions would go unpunished, in part, because of a Supreme Court decision that outlined protections for presidents from criminal prosecution if they are acting in their official capacity as commander-in-chief. 

“The court he created ended up being the court that helped to enable Donald Trump to not eventually face prosecution. It was the biggest miscalculation of his political career, and will no doubt be a stain on his legacy,” Mr. Tackett said. 

The battle to succeed Mr. McConnell in the Senate is already shaping up, and conservatives see it as an opportunity to get a tried-and-true ally of Mr. Trump into the chamber. Within an hour of the senator announcing he would step aside, former Kentucky attorney general Daniel Cameron — who recently lost a gubernatorial election in the state and was once an aide to Mr. McConnell — said Thursday that he will run. 

“Kentucky, it’s time for a new generation of leadership in the U.S. Senate. Let’s do this,” Mr. Cameron wrote in a post on X, including a photo of him with his wife and two children. A scroll through Mr. Cameron’s feed on X shows him effusively praising the president for nearly all of his decisions.

Congressman Andy Barr, an ally of House Republican leadership who also once worked in Mr. McConnell’s office, said in a statement Thursday that he also was considering a race for the Senate. CBS News reported that Mr. Barr privately told allies over the weekend that he would run for Senate if Mr. McConnell did not seek another term.