For Mr. Cassidy, it was a sense of outrage at the former president’s actions, starting long before the assault on Jan. 6, that played the dominant role. In an interview on Sunday, Mr. Cassidy said Mr. Trump had “trumpeted that lie” about the election for months, then sat by for hours as lawmakers and his own vice president were under attack in the Capitol and did nothing — other than to call Republican senators to ask them to continue challenging the election results.

“That anger simmers in the background,” Mr. Cassidy said. “My whole life, reading about great men and women who sacrifice for our country, who sacrifice so that we could have the freedoms that we have here today — and the idea that somebody would attempt to usurp those and destroy them?”

“It still angers me,” he continued. “It just angers the heck out of me.”

Many Republicans privately shared Mr. Cassidy’s rage, but the fact that only seven of them were ultimately willing to find Mr. Trump guilty underscored the extraordinary fealty the former president still commands in the party.