ORANGE CITY, Iowa — Representative Steve King tapped his fingers impatiently against the lectern as his Democratic debate opponent, Christie Vilsack, lit into him about the House’s failure to take up a farm bill. “I have just one question for Congressman King,” Mrs. Vilsack said, as the Republican looked on with irritation. “Where is the farm bill?”

It was an uncommon moment for Mr. King, the Tea Party firebrand who for the last decade has threshed his opponents like a combine through corn. This time around, he is in the unfamiliar and highly uncomfortable position of defending his House seat against an aggressive, well-known Democrat with a lot of outside money supporting her bid.

Mr. King’s troubles underscore the liability of a national profile built largely on Tea Party credentials and incendiary statements — his have included comparing illegal immigrants to domestic pets, defending dog fighting and likening Capitol Hill maintenance workers to “Stasi troops” after they were ordered to install new light bulbs — as candidates in newly drawn districts face an electorate tired of empty political talk and stalemate.