Democrats and Republicans alike privately agreed that it looked unlikely that even a single Republican would vote for impeachment when it reaches the House floor. In the Senate, Republican strategists said they believed they might lose two senators — Susan Collins of Maine and Lisa Murkowski of Alaska — while Democratic strategists said they also might lose two — Joe Manchin III of West Virginia and Kyrsten Sinema of Arizona.

“We’ve just had this partisan divide ever since the Clinton years,” said former Representative Barbara J. Comstock, Republican of Virginia. “Whether it was Supreme Court nominations or this, it’s just become a team sport, shirts and skins, no matter what the issues are.”

Steve Elmendorf, the top aide to the House Democratic leader when Mr. Clinton was impeached, agreed that lawmakers appear locked into their positions. “Except,” he cautioned, “we are in the Trump show, where anything can happen. Two months ago, we did not think he was going to be impeached over a phone call we knew little about.”