A fervent critic of Mr. Trump, Mr. Stevens has a book out this month, “It Was All a Lie,” which charts what he describes as the Republican Party’s decades-long spiral into racism, nativism and ultimately Trumpism. He said descendants of political families might feel burdens of virtue more acutely than others do. He mentioned George Romney’s protest against what he considered the party’s lack of commitment to civil rights at the Republican National Convention in 1964. “To me, you can take a direct line from there to Mitt saying ‘Black Lives Matter’ in 2020,” he said.

Until recently, no one would have ever described Mitt Romney as a leading dissident within his party. But the Trump years have imposed a distinct set of character tests upon elected Republicans at every level.

Mr. Romney idolized his father, to a point where he would write “Dad” on a sheet of paper at his lectern before big political speeches and debates. As he has navigated the lonely terrain of a defiant Republican in the Trump era, Mr. Romney said his father had loomed as a powerful example.

“It’s not as if I stop to consider ‘What would Dad want me to do?’” Mr. Romney said. As he has gotten older and embarked on what will most likely be the last chapter of his career, in the Senate, Mr. Romney said, it has become easier to heed the lessons of his forebears rather than the demands of political expediency.