Last year, the operation to defend the Supreme Court nominee against allegations of sexual assault was swiftly executed and well funded, bringing together the most disparate parts of the conservative movement.

It was also notably not a pro-Trump effort. Conservatives, many of whom have long harbored misgivings about the president’s conduct, united in defense of Justice Kavanaugh, whom they saw as one of their own — a trusted, churchgoing family man who had been smeared, with decades-old stories from his high school days, as part of a liberal plot to prevent the Supreme Court from tilting any further to the right.

Impeachment, on the other hand, is different in that it will likely serve as a referendum on Mr. Trump’s presidency, especially as it centers on allegations from earlier this year — not three decades ago. Ultimately, that means Republican members of Congress would have to decide whether they are comfortable saying, in effect, that it is perfectly fine for a president to use the power of the office to solicit foreign intervention in an election.