According to Congressional Quarterly, several Republican candidates might “face primary opponents drawing strength from the ‘tea party’ movement and its fervent opposition to anything that might trigger higher taxes or bigger government.”

[Mark] Souder [(R, Ind.)] first elected in the Republican wave of 1994, usually lines up with his party’s right wing. But he has angered some conservatives by endorsing earmarks for his district and by backing both the 2008 financial industry bailout (PL 110-343) and the expansion of the program to help recreational vehicle makers.
Rachel Grubb, a self-described “blue-collar mom,” is challenging Souder in the May primary, running on promises that she would strictly adhere to the Constitution and pay attention to those in the electorate “who say those in power have been there far too long.”

Others are in the same boat:

Other Republicans facing primary challenges from energized activists are Reps. Todd Akin of Missouri, Lee Terry of Nebraska and Bob Inglis of South Carolina.

The whole thing here.