Since he first outlined his belief that the special counsel’s report showed that Mr. Trump “engaged in impeachable conduct,” Republicans nationally have lined up in opposition to Mr. Amash, a five-term Republican with a reputation for saying “no.” From his earliest days in Washington, Mr. Amash has been a marginal figure, consistently voting against both political parties, publishing explanations of his latest “no” on a Facebook page where he proclaims, “I defend liberty and explain every vote here.”

Principled, yes, even critics will concede. A thought leader in the Republican Party? No.

But in Mr. Trump’s Republican Party, no figure is too marginal to be ignored — and even the faintest spark of opposition must be snuffed out, doused, crushed and buried.

Since Mr. Amash concluded that the president “engaged in impeachable conduct,” the billionaire DeVos family, whose money has helped bankroll the Republican Party here for decades, said through a spokesman that it would not financially support the congressman. Two Republicans have announced they will run against him in next year’s Republican primary race. And the president called him “a total lightweight who opposes me and some of our great Republican ideas and policies just for the sake of getting his name out there.”

“He’s isolated and marginalized himself in D.C.,” said Jim Lower, a state representative who rushed to put forward his primary challenge after Mr. Amash’s initial flurry of tweets about the president’s actions.

But in Grand Rapids, his political stronghold, Mr. Amash’s boldness was still applauded — wildly. Political strategists and voters say this congressional district in southwestern Michigan may not only tolerate an unabashed and frequent Republican critic of the president. It might also demand it. Mr. Amash’s event on Tuesday served to underscore the voter dynamics that will ultimately dictate his political future representing Michigan’s third district.