Mr. Grimm never did make that next term. He pleaded guilty to tax evasion and instead went to prison. But he said his case could provide a road map of sorts for Mr. Collins: Dig in, hire good lawyers (read: expensive), expect to be pummeled in the press and find something, anything else to run on and talk about.

“He’s going to have to swallow every bit of it. And smile,” Mr. Grimm said. “He should go right back to his job and remain professional.”

Which can be hard, especially when nearly every shouted question begins with indictment and ends with allegations. Mr. Grimm warned Mr. Collins that his old friends and allies — he was the first member of Congress to endorse Donald J. Trump in 2016 — will now give him the cold shoulder. The Capitol corridors will suddenly empty in his pathway.

“Washington, as long as you’re riding high, they want to be your friend,” Mr. Grimm said. “And when you’re not, they don’t want to be anywhere near you.” He said he had taken to heart the old saw of the only true friend in Washington is a dog, adopting one early in his first term.

The dog’s name is Sebastian. “Still part of the family,” Mr. Grimm said.

When Mr. Collins gets back to Washington, there will be a stern sit-down with the Republican leadership, Mr. Grimm said, but he advised Mr. Collins to not be overly concerned. He said that Speaker Paul Ryan, who already stripped Mr. Collins of his most plum committee assignment, has little more leverage to employ.