WASHINGTON — Representative Jean Schmidt of Ohio never had it easy.

Since her first House race, in 2005, Ms. Schmidt, a Republican, had endured tough primaries and had vexed members of both parties here more than once. Yet she was expected to prevail again on Tuesday and overcome a primary challenge from Brad Wenstrup, a doctor and Iraq war veteran who has never held political office.

But as Congressional election attention turned to the defeat of Representative Dennis Kucinich in another part of the state, Mr. Wenstrup, aided by a new “super PAC” that is targeting incumbents in both parties, dealt Ms. Schmidt a stunning upset. It was a warning shot to all incumbents, especially those who have run afoul of conservative voters over the last year.

On Wednesday, lawmakers and political experts were still weighing whether Ms. Schmidt’s loss meant that her political luck had simply run out or was a sign of things to come. But her defeat clearly illustrated both the power of third-party groups to successfully intervene in races where they may not have gone before and the threat to all incumbents in a country that is down — way down — on Congress.