The ad, titled "Corner Bar," portrays campaign contributors to Democratic challenger Bob Casey Jr. as criminals puffing on cigars in a jail cell. The ad does not name contributors but vaguely identifies each alleged contributor, such as a "New Jersey developer," and how much each one allegedly gave to Casey.

"It's a seriously flawed commercial that is over the top," says Terry Madonna, a political science professor at Franklin and Marshall College in Lancaster, Pa. "It leads you to the impression that this is Casey's campaign team and they are in jail."

The Casey campaign and several press reports have refuted the allegations made in the ad. The Philadelphia Inquirer says that the only contributions that any of the people alluded to in the ad have made for this Senate race were to Santorum. "One of the people supposedly on Bob Casey's 'team' has been dead for almost two years," says Larry Smar, spokesman for the Casey campaign.

The ad has dominated discussion on Pennsylvania political talk radio and TV shows, says Jerry Shuster, a political communications professor at the University of Pittsburgh. "It's creating quite a storm, and it's not a positive storm for Santorum," he says.

But Shuster warns that if Casey does not aggressively respond to the veracity of the ad, viewers may assume that the underlying premise is true, which could hurt Casey. "The electorate consistently says they don't like attack ads," says Shuster, "but they are often the kind of ads that work best."