The district is historically Republican, once represented by Gerald R. Ford before he was named Richard M. Nixon’s vice president. But an influx of younger voters and college graduates have shifted parts of the district leftward, causing some Democrats to wonder if the district could flip if Mr. Amash is bruised badly in a primary or defeated by a hard-right Trump supporter.
Possible redistricting, currently in the hands of the Supreme Court, could shift the demographics of the district even further.
The National Republican Congressional Committee does not get involved in primaries, a spokesman for the organization said. The Democratic Congressional Campaign Committee, the campaign arm for House Democrats, has not formally targeted Mr. Amash’s district, but an official said that the group was actively recruiting challengers for the general election.
Mr. Lower said that he planned to focus not only on the congressman’s stance toward the president’s conduct, but also on what he deemed to be a weak legislative record. Mr. Lower, along with Tom Norton, a National Guard veteran, have both filed to challenge Mr. Amash, who did not have a primary opponent in 2018.
“I don’t think the district is going to buy into it, frankly,” Mr. Lower said of Mr. Amash’s explanation of the president’s behavior as reaching “the threshold of impeachment.”
But Mr. Amash has been here before. In 2014, business-minded Republicans began a series of primary challenges against black-sheep Republicans who they believed were giving the party a bad name. In the far suburbs of Detroit, Kerry Bentivolio, sometimes a Santa Claus impersonator, lost. Across the state, Mr. Amash won by nearly 15 percentage points — and demanded an apology from his vanquished Republican rival.
His allies — and grudging admirers — say that voters have come to expect Mr. Amash, who considers himself a libertarian and strict constitutionalist, to choose principle over party. And Mr. Amash waved off concerns that his independent streak would cost him re-election, telling attendees, “You have to do the right thing regardless.”