WASHINGTON — As he weighed the evidence against President Trump, Senator Lamar Alexander reached an unavoidable conclusion: Mr. Trump had done what he was accused of, pressuring a foreign power to investigate his political rival. But however inappropriate his conduct, another conviction overrode the first: Americans would not tolerate the Senate stepping in to substitute its own judgment for that of the voters fewer than 10 months before the next election.

“The Senate reflects the country, and the country is as divided as it has been for a long time,” Mr. Alexander said Friday during an interview in his Capitol office. “For the Senate to tear up the ballots in this election and say President Trump couldn’t be on it, the country probably wouldn’t accept that. It would just pour gasoline on cultural fires that are burning out there.”

With that logic, Mr. Alexander delivered a victory to Mr. Trump — and to Senator Mitch McConnell, Republican of Kentucky and the majority leader, with whom Mr. Alexander has been friends for more than a half-century. In announcing he would vote to block witnesses at Mr. Trump’s impeachment trial, he set Mr. Trump on a quick course to his inevitable acquittal.

Many Republicans appeared to be following Mr. Alexander’s lead on Friday, saying the Tennessee senator had echoed the feelings of their caucus — and the country.