Democrat Bob Casey maintained a slight lead over Republican Sen. Rick Santorum in Pennsylvania, but more voters say they have a poor opinion of the challenger, according to an independent poll released Thursday.

In a three-way contest, the new Keystone Poll found Casey leading Santorum 44 percent to 39 percent among registered voters. Green Party candidate Carl Romanelli had 4 percent. Thirteen percent were undecided in the poll, which was conducted by Franklin & Marshall College between Aug. 16-21.

In May, before Romanelli entered the race, the same pollster had Casey with a six-point lead over Santorum, the No. 3 Senate Republican.

Twenty percent said they had an unfavorable opinion of Casey, a seven-point increase. The number with a favorable opinion of him was virtually the same at 31 percent. Forty-nine percent didn't know or were undecided.

The number with either a favorable or unfavorable opinion of Santorum remained about the same at 37 percent each. Twenty-six percent didn't know or were undecided.

It appears Santorum has had success in placing Casey, the state treasurer, in a bad light, even as the opinion of Santorum hasn't shifted much, said G. Terry Madonna, the Franklin & Marshall professor who directed the poll.

"Casey has to define himself before Santorum defines him completely," Madonna said.

With two months to go before the election, the Casey campaign said Wednesday that he would go statewide with television ads on Thursday. Santorum, who has been running ads statewide since late June, unleashed his first negative ad against Casey on Wednesday.

The results were based on telephone interviews with 551 Pennsylvanians. The poll carried a sampling margin of error of plus or minus 4 percentage points.

The survey was commissioned by the Philadelphia Daily News, The Patriot-News in Harrisburg, WGAL-TV in Lancaster, the Pittsburgh Tribune-Review and WTAE-TV in Pittsburgh.