People close to Ms. Cheney, who insisted on anonymity so they could discuss her private views, said that her break with the pro-Trump faction reflected her belief that many more Republicans share her disgust with how seriously Mr. Trump undermined confidence in the country’s electoral system.

As she watched Mr. Trump and his supporters peddle conspiracy theories and promote what she called “the big lie,” Ms. Cheney became deeply unsettled by how many of her colleagues seemed so cavalier about Mr. Trump’s actions, friends and associates said. She was also bothered by the way Republicans cheered and mimicked the kind of behavior she expected of a foreign authoritarian leader but never from an American president…

Ms. Cheney’s allies described her as at peace with the stance she has taken on Mr. Trump. Representative Adam Kinzinger of Illinois, one of the nine other Republicans who voted to impeach Mr. Trump, said that too many of his colleagues were doing the opposite of what Ms. Cheney is.

“They’re waiting to see if Trump collapses,” he said. “And then if he does, they’ll be like, ‘I’ve never been with Trump, ever.’” He described the effort to punish Ms. Cheney as “cancel culture on the right.”