When undecided "leaners," or voters who don't initially commit to voting for either candidate but express a preference, are added, Casey leads by 12 points, 55 to 43 percent. Rasmussen found similar polling results last week, with a margin of 13 points, although Casey was leading Santorum 50 percent to 37 percent.

Last week, Rasmussen changed its ranking of the race from "Leans Democrat" to "Democrat." Santorum is the only Republican incumbent to have his re-election chances rated by the firm as weaker than "Leans Democrat."

"Both candidates attract about four fifths of their partisan base," says the independent polling firm. "But Casey has a 3-to-1 advantage–72 percent to 25 percent–among unaffiliated voters." Among all voters, Santorum is viewed favorably by only 46 percent, says the poll. Casey, on the other hand, gets "thumbs up" from 57 percent of all voters.

–Danielle Knight