White House officials say they have learned a few things about party unity since then. With the House vote as well as a Senate trial all but foregone conclusions, the goal from now on will be to keep Republicans in lock step with the president — and on message — as impeachment runs its course.

To make sure that happens, a charm offensive to keep congressional Republicans close started months ago. It has included movie nights in the East Wing, lunches and dinners at the White House with the president, and trips to Camp David. Mick Mulvaney, the acting chief of staff, has hosted 56 House Republicans at the retreat in recent weeks.

“They come out, and we’re all on the same page,” Mr. Gidley said. “And one of the reasons they’re doing it, they’ll tell you, is that the messages work in their districts — whether right leaning, left leaning or down the middle.”

But Joe Lockhart, the White House press secretary during the impeachment of President Bill Clinton, said he saw some immediate problems with the charm offensive, which he said dealt hardly at all with the substance of the charges against Mr. Trump and instead placed an emphasis on attacking the impeachment process itself.

“Are they doing a good job of disseminating false information? Absolutely,” Mr. Lockhart said of Republican lawmakers. “When you’re arguing process, most of the time you’ve already lost the argument.”

The president and his allies have closely followed Republican polling that they say shows Mr. Trump winning in several states that would be crucial for his re-election, and they are using those numbers as evidence of a shift in public opinion.

Still, Celinda Lake, a longtime Democratic pollster, said national polling on impeachment had remained relatively static through the impeachment inquiry, with some 50 percent of voters favoring it and 42 percent opposing it.