WASHINGTON — At a White House event in November 2019, President Donald J. Trump offered unrestrained praise for one person on hand he regarded as singularly responsible for his administration’s remarkable record of placing conservatives on the courts.

“The nation owes an immense debt of gratitude to a man whose leadership has been instrumental to our success,” Mr. Trump said.

That man was Senator Mitch McConnell, now enmeshed in an ugly feud with the former president that has significant ramifications for the future of the Republican Party. The rift is extraordinary partly because perhaps no one did more to advance Mr. Trump and his Washington ambitions than Mr. McConnell, who had ambitions of his own and saw Mr. Trump as a vessel to pour them in.

“Trump would not have been able to achieve his objectives without a strong Senate leader,” said Karl Rove, the Republican strategist and former political adviser to President George W. Bush.

The relationship had its rocky moments but was usually cordial enough — until it went extremely bad in recent days as Mr. McConnell excoriated Mr. Trump on the Senate floor after acquitting him in an impeachment trial and Mr. Trump responded with a cutting personal broadside. It was a messy breakup years in the making.

Like most Americans, Mr. McConnell expected Mr. Trump to lose to Hillary Clinton in November 2016, and he also braced for the potential loss of the Senate majority as party pollsters and strategists predicted a big night for Democrats. Much to the surprise of Mr. McConnell, Republicans held on and Mr. Trump triumphed, an outcome for which Mr. McConnell could deservedly take some credit.

A strong argument can be made that Mr. McConnell, by preventing President Barack Obama from filling the Supreme Court vacancy created by the death of Justice Antonin Scalia in February 2016, cleared Mr. Trump’s path to the White House.

The sudden political focus on the court provided a way for Mr. Trump to assure conservatives wary of his character flaws that he could be their champion. He and his legal advisers assembled a now famous list of potential conservative nominees that he promised he would choose from to calm evangelicals and others on the right who worried he might appoint a more liberal justice to succeed Justice Scalia.

Mr. Trump himself recognized the political power of that list and the Scalia vacancy as he lavished praise on Mr. McConnell that day at the White House.

“It really did have an impact on the election,” Mr. Trump said at the celebration in the East Room. “People knew me very well, but they didn’t know, ‘Is he liberal? Conservative?’”

Mr. McConnell, the canny Senate leader, and Mr. Trump, the Washington novice suddenly ensconced in the White House, became a team. It was not a great personal match. Mr. McConnell spilled nothing of his intentions; Mr. Trump spilled all.

Mr. Trump could not relate to the buttoned-lip approach of Mr. McConnell as he made clear this week in his scathing statement describing Mr. McConnell as “dour, sullen and unsmiling.” Mr. McConnell held private disdain for Mr. Trump and saw a flawed personality with a sketchy history who was not at all versed in the customs and rites of Washington.

But as the Trump era opened, Mr. McConnell was just happy that Mr. Trump didn’t turn out to be a Democrat, though some congressional Republicans were not so sure. And it didn’t hurt that Mr. Trump brought on Mr. McConnell’s wife, Elaine Chao, as transportation secretary.

“Back during the campaign, there were a lot of questions: Is Trump really a conservative? A lot of questions about it,” Mr. McConnell told The New York Times in February 2017 as the chaotic White House set up shop. “But if you look at the steps that have been taken so far, looks good to me.”

As he looked, Mr. McConnell, long obsessed with the federal courts, saw opportunity. Even before Mr. Trump was sworn in, Mr. McConnell approached Donald F. McGahn II, the incoming White House counsel, about establishing an assembly line of judicial nominees to fill vacancies caused by Republicans’ refusal to consider Obama administration nominees.

The interests of the Trump administration and Mitch McConnell had aligned. He prioritized appeals court judges, eliminated the 60-vote threshold for Supreme Court nominees and stood by Justice Brett M. Kavanaugh despite accusations of sexual misconduct. He pushed Justice Amy Coney Barrett just days before the 2020 presidential election despite using the approach of the 2016 election to block Judge Merrick B. Garland’s nomination eight months before the voting. The judicial success provided both the president and the Republican leader with a legacy.

But it wasn’t just judges. Mr. McConnell delivered Mr. Trump’s tax cuts, remained stoic during regular presidential outbursts and made short work of the 2020 impeachment, with his most prominent failure in conservative eyes being the inability to overturn the Affordable Care Act.

“Mitch McConnell was indispensable to Donald Trump’s success,” Senator Lindsey Graham, Republican of South Carolina and an occasional go-between who is traveling to meet Mr. Trump this weekend in Florida to try to smooth things over, said on Fox News. “Mitch McConnell working with Donald Trump did a hell of a job.”

Then came the election. Mr. Trump refused to accept the results, making wild and unsubstantiated claims of voter fraud. Mr. McConnell indulged him and refused to recognize President Biden as the winner until he could avoid it no longer after the states certified their electoral votes on Dec. 14. He congratulated Mr. Biden the next day.

The interests of Mr. McConnell and Mr. Trump now sharply diverged, with Mr. McConnell fixated on regaining power in 2022 while Mr. Trump was stuck on 2020, making outlandish allegations that threatened to drive off more suburban voters and imperiled two Georgia seats that went to Democrats on Jan. 5. Then the riot the next day found marauders in the Senate chamber, Mr. McConnell’s sanctum sanctorum.

“This mob was fed lies,” Mr. McConnell declared on Jan. 19, accusing Mr. Trump of provoking the rioters and prompting rumblings that he of all people might vote to convict Mr. Trump in the coming impeachment trial. But he did not. Instead, he voted to acquit Mr. Trump then tried to bury him minutes later while distinguishing between Mr. Trump’s responsibility for the riot and the Trump voters Mr. McConnell and Republican Senate candidates would need next year.

“Seventy-four million Americans did not engineer the campaign of disinformation and rage that provoked it,” Mr. McConnell said. “One person did. Just one.”

Mr. Rove said Mr. McConnell handled it well.

“McConnell reads his conference and he knows that, like him, they thought simultaneously that this was a highly partisan process and not good for country, but also that Trump had played a significant role in fomenting Jan. 6,” he said.

Then it was Mr. McConnell doing the provoking. His post-trial speech and a subsequent op-ed in The Wall Street Journal drew the ire of Mr. Tump, who fired back with a call for Republicans to dump their leader — an unlikely prospect — and a threat to mount primary challenges against candidates allied with Mr. McConnell, a more worrisome prospect for members of the party.

Now the question is whether Mr. Trump will follow through, causing intramural fights that ultimately lead to Democratic victories. Mr. McConnell’s allies note that he has been in this position before facing challenges from the right and came out on top.

“My money,” said Bob Stevenson, a former top Senate Republican leadership aide active in Senate races, “is on Mitch.”