But Mr. McConnell, prioritizing elections over policy, calculated that by blocking or delaying Democratic legislation, above all through aggressive use of the filibuster, Republicans would create a tedious gridlock that voters would blame the Democrats for. After all, weren’t they the ones in power?

Mr. McConnell was right. This strategy helped to foment opposition to the health care bill, and to drive huge Republican gains in the 2010 election. But it also fueled the rise of the Tea Party, which was motivated substantially by the notion that Mr. Obama was “ramming things down our throats” — that is, passing legislation on a partisan basis after Mr. McConnell withheld any Republican negotiation. Of course, Mr. McConnell proceeded to have plenty of headaches managing the far-right contingent in his own caucus, but it was a contingent he helped produce.

His role in the election of Mr. Trump was even more direct. Most notable was his refusal to hold a confirmation hearing, let alone a vote on Merrick Garland, Mr. Obama’s nominee to replace Antonin Scalia on the Supreme Court, despite the fact that the nomination was made a full 10 months before the end of Mr. Obama’s term. This refusal exploded norms and dismayed Beltway arbiters who had long accepted Mr. McConnell’s claim to be a guardian of Washington institutions. It also provided crucial motivation to Republicans who had grave qualms about Mr. Trump but were able to justify voting for him as “saving Scalia’s seat.”

Mr. McConnell’s other form of aid for Mr. Trump was more hidden. As The Washington Post reported a month after the 2016 election, Mr. Obama had been prepared that September to go public with a C.I.A. assessment laying bare the extent of Russian intervention in the election. But he was largely dissuaded by a threat from Mr. McConnell. During a secret briefing for congressional leaders, The Post reported, Mr. McConnell “raised doubts about the underlying intelligence and made clear to the administration that he would consider any effort by the White House to challenge the Russians publicly an act of partisan politics.” The Obama administration kept mum, and voters had to wait until after Mr. Trump’s election to learn the depth of Russian involvement.

Now, with the retirement of Justice Anthony M. Kennedy, it is evident just how much of a lasting legacy Mitch McConnell’s will leave the country: Donald Trump will have at least two lifetime appointments to the Supreme Court. The president has — and will now enjoy — greater latitude in filling those seats as a result of Mr. McConnell’s doing away last year with the 60-vote requirement for Senate confirmation, to get Neil Gorsuch seated. In the day and a half before Justice Kennedy’s announcement, the impact of the Scalia seat was made plain again, as the court issued 5-4 rulings in favor of Mr. Trump’s “travel ban” and anti-abortion groups, and against public employee unions.

The abortion and union rulings had an ironic resonance, as far as Mr. McConnell goes. In the 1970s, when he ran for county executive in Louisville, he secured the pivotal endorsement of the A.F.L.-C.I.O. by pledging to back collective bargaining for public employees (a promise that went unfulfilled), and while in office he worked effectively behind the scenes to protect abortion rights locally.