CEDAR RAPIDS, Iowa — It is still uncertain whether Republicans in Congress will succeed in undoing the Affordable Care Act, but the debate over repealing it may have already done in Midwestern Nice.

It was Tuesday evening, and inside a community college gymnasium, the jeers were hailing down on Representative Rod Blum, a Republican from northeastern Iowa, as he defended his vote for a bill that would reshape health care and repeal much of President Barack Obama’s biggest domestic accomplishment.

“Be an adult!”

“Hey, shut up!”

“Answer our question!”

Few Republicans are holding in-person town hall events in the wake of the bill’s narrow passage in the House last week, but Mr. Blum’s constituents were not giving him any credit for showing up. They waved red sheets of paper to signal their disapproval and hurled down a chorus of criticism over health care, President Trump’s taxes, the investigations into Russian meddling in the presidential election and the latest bombshell: the firing of the F.B.I. director, James B. Comey.

“I’m not going to over-shout you, so if you want to keep interrupting,” Mr. Blum started to say.

“Booo!”