In a presidency without precedent, mark another moment for the history books. While the accusation of treason has been thrown around on the edges of the political debate from time to time, never in the modern era has it become part of the national conversation in such a prominent way.

To the president’s defenders, this all sounds like another eruption of what they often call Trump Derangement Syndrome. That he drives his critics to such extremes, they argue, says more about them than it does about Mr. Trump. As the president backtracked on his deferential comments at Monday’s meeting with Mr. Putin and asserted that he really does accept that Russia intervened in the 2016 election, allies assumed that this, too, would blow over.

But the list of Republicans rebuking the president included not just the usual suspects like Mr. Corker, who has been a frequent critic and plans on retiring when his term is up in January, but friends of the president like the former House speaker Newt Gingrich, who called his performance in Finland “the most serious mistake of his presidency,” and the conservative editorial page of The Wall Street Journal, which called it a “national embarrassment.”

Even some of the normally friendly folks at Fox News expressed astonishment, including Neil Cavuto and Abby Huntsman, whose father, Jon Huntsman, is Mr. Trump’s ambassador to Moscow.