Democrat Bob Casey is holding onto his lead over Republican Sen. Rick Santorum in one of the nation's hottest Senate races, despite the recent appearance of a Green Party candidate who is considered a spoiler for Casey, a poll released Tuesday shows.

The poll also shows, though, that Santorum has narrowed the gap from an 18-point difference in June.

In a three-way contest, the new poll found Casey leading Santorum 45 percent to 39 percent, with Green Party candidate Carl Romanelli picking up 5 percent. Eleven percent said they were undecided or did not plan to vote, according to the Quinnipiac University poll.

As a two-way race, without Romanelli, 47 percent of the same respondents favored Casey to 40 percent for Santorum, with 13 percent undecided or expressing another opinion.

The results of the poll, based on telephone interviews with 1,384 Pennsylvania registered voters from Aug. 8 through Monday, carry a sampling margin of error of plus or minus 2.5 percentage points. The June poll showing Casey leading Santorum by 18 points, 52 percent to 34 percent, was also conducted by Quinnipiac University.

The Casey campaign said Tuesday it expected the race to tighten.

"Obviously there was a reason Rick Santorum and his supporters bankrolled the petition drive, why his staff was so eager to collect petitions and encourage Romanelli's candidacy," said Larry Smar, Casey's campaign spokesman.

Romanelli, a railroad industry consultant from Wilkes-Barre and a former family court support officer, submitted enough signatures Aug. 1 to get on the November general election ballot. He has said most of his financial backing came from Republicans.

Some Democratic voters could find Romanelli an attractive choice because he supports a woman's right to an abortion, while Casey, currently the state treasurer, and Santorum do not.

"The voters are looking at the totality of the race. They're really paying attention for the first time," said Clay F. Richards, assistant director of the Connecticut-based university's polling institute.

Romanelli said he was pleased with the poll results because normally Green Party candidates poll at 2 percent or less and "we haven't even really gotten started yet."

Santorum's campaign manager, Virginia Davis, said "it's hard to ignore that some of these polls are showing a trend in favor of Santorum."

But Davis said the campaign doesn't expect to see "a real dramatic shift until voters have the opportunity to see the candidates debate." So far, only one debate is scheduled, on NBC's "Meet the Press" over Labor Day weekend. Santorum has called for 10 debates.