The latest ad depicts David Leinbach, described as a small-business owner, who claims that Casey supports raising taxes that would harm small-business owners. "We need a strong economy, not higher taxes," it says. The ad was paid for by Americans for Job Security, a Virginia-based association that will not reveal its members. This is the fifth television advertisement the group has run on the race since last November.

The Casey campaign argues that Casey does not support raising taxes on small businesses—but that he does oppose President Bush's tax cuts for the wealthy. "AJS is doing everything possible to keep Santorum in this race," says Larry Smar, a spokesman for Casey. "When will someone hold them accountable and make them release their donors?" Smar said the group is backed by the insurance and pharmaceutical industry and has connections to the Swift Boat Veterans for Truth, the group that opposed John Kerry for President.

Mike Dubke, president of AJS, told U.S. News that it won't reveal its members because it was criticized when it revealed that the American Insurance Association was one of its first backers in 1997. Since then, the group has asked the association not to rejoin, said Dubke. He said the AJS's lawyer is Ben Ginsberg, who resigned from the Bush-Cheney campaign after it was disclosed that he provided legal advice to Swift Boat Veterans for Truth. "I guess there is a connection with six degrees of separation," says Dubke.

Dubke said his group was only trying to further the debate in a hotly contested race. AJS has run ads in more than 40 states since its founding in 1997. "We see our value as making public officials talk about issues," says Dubke.

Virginia Davis, spokeswoman for the Santorum campaign, says that it has no communication with AJS. "The larger question here is: Would Casey have supported the tax cuts passed in 2001 and 2003?" says Davis.

–Danielle Knight