Senator McConnell, in announcing he will step down as Senate Republican leader come November, is setting up what will likely be a hotly contested campaign to succeed him. As the longest-serving Senate leader in American history, there is a deep bench of experienced, ambitious legislators who are eager to take over the top job.

“I turned 82 last week. The end of my contributions are closer than I’d prefer,” Mr. McConnell said on the Senate floor on Wednesday, announcing his retirement.

“Father Time remains undefeated. I’m no longer the young man sitting in the back, hoping my colleagues would remember my name. It’s time for a new generation of leadership,” he added.

Mr. McConnell, in recent years, has been a foil for the more Trump-aligned House Republican conference and the new generation of senators who entered the upper chamber since the former president’s election in 2016. 

The Kentuckyian just recently worked hand-in-glove with Senator Schumer to pass a foreign aid bill that would provide nearly $100 billion in aid to countries like Israel, Ukraine, and Free China. For doing that, Senator Vance called Mr. McConnell “a failure” of a conservative. 

Yet Mr. McConnell in his Wednesday remarks highlighted his support for these foreign aid projects, saying, “For as long as I am drawing breath on this earth, I will defend American exceptionalism.”

Conservative and liberal writers and activists alike feel no love lost for the departing Senate leader. 

The executive director of the American Conservative, Curt Mills, said Mr. McConnell leaves behind a “destructive legacy,” and described the senator as a “secretive figure of pure, uncharismatic ego.”

An anti-Trump former Republican operative, Tim Miller, said Mr. McConnell is responsible for helping advance the “anti-democratic” sentiments in the GOP today. 

“Had McConnell retired in January 2021 and led the conviction of Donald Trump, his legislative legacy would’ve been about the same, and he would’ve saved the country from the anti-democratic threat he saw clearly,” Mr. Miller said. “We would not be on the precipice we are on now if not for his cowardice.”

Mr. McConnell announced he would stay in the Senate through the remainder of his term, which ends in 2026. He did not say if he will run for re-election. 

The campaign to succeed Mr. McConnell as Republican leader began long ago, when senior senators began floating names of potential heirs. 

The most prominent figures in the Senate likely to run to replace the leader include “the Three Johns,” as they are known — Senators Barrasso, Cornyn, and Thune. 

All three men have at one time served on Mr. McConnell’s leadership team. Mr. Cornyn served as Republican whip between 2013 and 2019, and Mr. Thune has held the role since. Mr. Barrasso has been chairman of the Republican Conference since 2019. 

All three men will have to mollify the right wing of the conference, especially as most senators come around to supporting President Trump for the 2024 election. 

Mr. Thune has the most complex relationship with the former president of the Three Johns. After the January 6, 2021, attack on the Capitol, Mr. Thune voted to acquit the former president at his Senate trial but said Mr. Trump had made “inexcusable” efforts to “undermine faith in our election system and disrupt the peaceful transfer of power.”

When Mr. Thune was up for re-election in 2022, Mr. Trump actively looked for a potential Republican challenger to put up against the South Dakotan. Mr. Trump reportedly asked his close ally, Governor Noem, to challenge her state’s senior senator, though she instead sought re-election. According to NBC News, the Trump team put together polling to show that she could defeat Mr. Thune in a Republican primary. 

When Mr. Thune endorsed Mr. Trump on Sunday, he did so half-heartedly, saying that he was prepared to support the former president only because he was clearly going to be the Republican nominee following his victory in the South Carolina primary. 

Mr. Cornyn has rarely offered any criticism of Mr. Trump and could be an ally should the 45th president return to the White House. He has, though, won enemies on the right after supporting a comprehensive bipartisan gun control bill in the Senate following the shooting at Robb Elementary School in his home state of Texas. 

Like Mr. Thune, Mr. Cornyn also belatedly endorsed Mr. Trump after the New Hampshire primary, saying that it was clear he would be the presidential nominee.  

Mr. Barrasso, on the other hand, has proven himself a fierce ally of the former president, in style and in policy. Of the Three Johns, Mr. Barrasso was the only one to vote against the foreign aid bill Mr. McConnell had helped write with Mr. Schumer. 

He was the first member of Senate Republican leadership to endorse Mr. Trump. “We need a strong Republican conservative House and Senate and we need Donald Trump back in the White House,” he told Fox News before the Iowa caucuses in January. 

He also won some friends in the former president’s orbit when he refused to endorse Congresswoman Liz Cheney in her 2022 Republican primary in their native Wyoming. Mr. Barrasso said she was only being supported by Democrats and would never survive the former president’s ire. 

One outside candidate who may make a run for the job is Senator Scott, who previously challenged Mr. McConnell for the leader position after the 2022 midterms. Mr. Scott garnered significant support from conservatives within the conference, including Senators Cruz, Vance, Johnson, Lee, and others. 

Mr. Scott did make enemies among his fellow Republicans after he failed to retake the Senate as chairman of the National Republican Senatorial Committee during the 2022 midterms.

One source close to Mr. Trump says that Mr. Scott is “probably” the best partner for the 45th president but doubted he could ever win over those senators who resent him for both losing a Senate seat in 2022 and subsequently challenging Mr. McConnell.