In an even, measured voice, belying the outrage she feels, Ms. Cheney has confronted the leader of her party and called out those who enabled him, becoming Mr. Trump’s most prominent antagonist even as the Justice Department takes its time considering what to do and President Biden largely sits on the sidelines. She has become the unlikely hero of many who once vilified her family and a pariah to fellow Republicans she once worked closely with, possibly sacrificing her political career in the process.

“I don’t look at it through a political lens,” she said in an interview this week in between drafting statements for Thursday’s hearing. “I look at it through the angle of: People need to understand how dangerous he is and how unfit for office he is.”

“I believe this is the most important thing I’ve ever done professionally,” she added, “and maybe the most important thing I ever do.”…

“I’m sure it’s as weird for them as it is for me,” Ms. Cheney said of spending so much time with Democrats. She has grown close to some of them, especially Representative Jamie Raskin of Maryland, and several have told her they cannot wait for the day when they can disagree with her again.

“That’ll mean our politics have righted themselves,” she said.