This is the day after the day after Democrats held the late Jack Murtha's house seat, and as the Wall Street Journal points out here, Republicans who thought the November elections were merely a formality are spooked. Many conservative pundits are attempting to explain away this defeat and in reality it wasn't as winnable as Republicans thought, especially with the Specter/Sestak primary drawing out dem voters..
But perhaps that's the lesson here; beware overconfidence. Republicans were unrealistic about this seat, which means it's entirely possible they're overconfident about this fall. It's a lot like a college team ranked #1 in the pre-season. You still have to go out and win every game to stay there. Republicans now know they're not invincible this fall. If they learn it, that could be a valuable lesson.