Washington ? 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 and 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.

With some two months to the election, Santorum launched his first negative ad Wednesday, criticizing Casey’s attendance record at his state treasurer job while the challenger was campaigning.

On Thursday, Casey answered with a critical ad that assailed Santorum for voting 98 percent of the time with President Bush, voting to give himself a pay raise and voting against raising the minimum wage.