There’s an obvious explanation for this cover-up: With the exception of Mr. Bolton and a few other key figures, everyone who has provided incriminating evidence about Mr. Trump has done so in public and under oath; and everyone who he claims would exonerate him has either refused to testify or been blocked from testifying by Mr. Trump. Mr. McConnell, calculating that any testimony would be very bad for the president and thus for the Republicans’ Senate majority, persuaded his colleagues to take their lumps and vote no on witnesses.

The precedent this sets is alarming enough: the Senate abandoning its role as the ultimate guard against a dangerous president. Just as bad is the rationale on which most Republicans have settled for refusing to hear from witnesses — that whatever you think of Mr. Trump’s behavior, it wasn’t impeachable, and there is no evidence that could change their minds.

Given the seriousness of the charges against Mr. Trump, it’s hard to envision anything that this president could do that would require Republican senators to vote for his removal.

There is one apt criticism leveled by Republicans, even if they have made it in bad faith: Democrats in the House of Representatives moved too fast in the impeachment process, voting before they could hear from key witnesses like Mr. Bolton. They justified this by pointing to the urgency of the situation. Mr. Trump has accepted foreign assistance to win one election, has actively sought it out for another and has given no indication that he plans to stop doing so. He also pledged to stonewall any congressional inquiry into his behavior, which could have led to months or years of litigation over witnesses and documents.

But would that have been worse than where we are now? Had the House kept the impeachment inquiry open on the grounds that it could not vote until it had completed its investigation, the spotlight would have remained on Mr. Trump and his corrupt behavior. Evidence would have continued to come out, and the American people would get the fullest possible picture of the president’s behavior.

Instead, with the Senate’s blessing for his scheming to have Ukraine investigate the Bidens, Mr. Trump now poses an even greater threat to the next election.

Senate Republicans’ indifference to the overwhelming public support for calling witnesses was of a piece with the party’s minority politics. Its president lost the popular vote by three million votes. Its Senate majority represents 15 million fewer Americans than the Democrats’ minority. In states like North Carolina, it rigs the maps to turn popular-vote losses into legislative majorities, then strips power from duly elected Democratic leaders.