There's a new Rasmussen Poll out for July. It shows Bob Casey up 50 to 39%.  This is a narrowing of four points from the one a month ago.

Santorum is viewed favorably by 44%, unfavorably by 50%; 30% view him "very" unfavorably. He is popular enough with his base, with 76% of Republicans viewing him favorably, 48% "very favorably" (numbers that have been increasing). But 47% percent of Democrats view him "very unfavorably." And Santorum barely scrapes up a 32% favorable rating from unaffiliated voters, the group he must win over to squeak through. Fifty-nine percent (59%) of unaffilateds and 61% of moderates view him unfavorably.

Casey is viewed favorably by 66% of all voters. That figure includes 57% of unaffiliated voters and 71% of moderates. But relatively few regarding him "very" favorably, confirming the perception that the election is largely a referendum on Santorum.

More here. (Including trends...)