Supporters of Santorum gave tens of thousands of dollars to finance a petition drive that allowed Carl Romanelli, a Green Party candidate, to qualify for the race, according to published reports. Romanelli, who is pro-abortion rights and against the Iraq war, could siphon liberal votes from Casey, who opposes abortion and is against setting a timetable for pulling troops out of Iraq.

The Santorum campaign says it encouraged Santorum supporters to help the Green Party get on the ballot. "Certainly, Senator Santorum and Carl Romanelli are on opposite sides of the spectrum," says Virginia Davis, a spokeswoman for the Santorum campaign. "But the bottom line is that we welcome any candidate who wants to debate. We welcome their participation."

The Casey campaign says Santorum, who is trailing Casey in the polls, is using the Green Party as a political pawn. "Rick Santorum is afraid to face Bob Casey one-on-one so he directed his donors to fund the Green Party," says Larry Smar, a spokesman for Bob Casey, currently the state treasurer. "Santorum has learned a lot of dirty tricks, and he has now proven why he embodies everything that is wrong with Washington."

—Danielle Knight