A survey by The New York Times on the eve of the trial found that 27 Republican senators had expressed opposition to charging Mr. Trump or otherwise holding him accountable by impeachment. Sixteen Republicans indicated they were undecided, and seven had no response. Most of those opposed increasingly fell back on process-based objections, rather than defending Mr. Trump.

“Why are we doing this?” said Senator Ron Johnson, Republican of Wisconsin. “I can’t think of something more divisive and unhealing than doing an impeachment trial when the president is already gone. It’s just vindictive. It’s ridiculous.”

Lawmakers in both parties cautioned that Republicans’ mood could quickly shift in the weeks ahead, if more evidence broke into public view about Mr. Trump’s actions or he provoked them further with his defiant threats of retribution.

Already, unflattering new details were surfacing about Mr. Trump’s broader campaign to use his power stay in office at any cost.