Even with a Green Party candidate now in the race, Democratic challenger Bob Casey leads Republican Sen. Rick Santorum, according to a poll released today.

In a three-way matchup, Casey leads Santorum 45 percent to 39 percent, with Green Party candidate Carl Romanelli picking up 5 percent, according to a Quinnipiac University poll of registered voters. Eleven percent were undecided or did not plan to vote.

Mr. Romanelli, a railroad industry consultant from Wilkes-Barre and a former family-court support officer, qualified with enough signatures Aug. 1 to get on the Nov. 7 general election ballot. He has said that most of his financial backing has come from Republicans, and Democrats have challenged his signatures in court.

Mr. Romanelli is considered a potential spoiler against Mr. Casey, who is now Pennsylvania's state treasurer, because he supports abortion rights while Mr. Santorum and Mr. Casey oppose them.

With less than three months to go before Election Day, Mr. Santorum has already begun a heavy schedule of statewide TV ads in one of the nation's most hotly contested Senate races. Mr. Casey's ads have so far focused on the Pittsburgh market.

In a two-way race, 47 percent of the respondents in the Quinnipiac Poll favored Mr. Casey to 40 percent for Mr. Santorum, with 13 percent either undecided or expressing another opinion. The seven-point advantage is down from an 18-point lead Mr. Casey had in the same poll in June.

Most of the ground Mr. Casey lost went to Mr. Romanelli, while Mr. Santorum's numbers improved slightly, said Clay F. Richards, assistant director of the Connecticut-based university's polling institute.

"The voters are looking at the totality of the race. They're really paying attention for the first time. The race doesn't really start until after Labor Day," Mr. Richards said. "This is a much more normal look at the Senate race. They are the kinds of numbers you would expect rather than polls that showed Santorum down by 20 points."