Republican Senator Rick Santorum continues to be the nation’s most vulnerable incumbent and has lost ground once again to Democratic challenger Bob Casey, Jr. Casey now leads the race, 50% to 37% (see crosstabs). If undecided voters leaning toward a candidate are added to the mix, Casey leads 52% to 39%.

We are now shifting this race from “Leans Democrat” to “Democrat” in our Senate Balance of Power summary. Santorum is the only Republican incumbent to have his re-election chances rated weaker than “Leans Democrat.” The weakest Democrat of 2006 is New Jersey’s Bob Menendez whose race with Tom Kean Jr. (R) is rated as a Toss-Up.

Most Casey supporters (60%), including 53% of Democrats, profess to be casting a vote against Santorum rather than a vote for Casey (31%). This is not too surprising given that Senator Santorum is viewed "very unfavorably" by 37% of all voters. Additionally, many Democrats in the state do not share Casey’s pro-life view on abortion.

Seventy-seven percent (77%) of Santorum supporters say their vote is "for Santorum" rather than "against Casey" (15%).

The survey also found that 52% of all Pennsylvania voters believe American troops should be brought home from Iraq within the next year; 32% disagree.

Santorum has been trailing in this race all year, at times by more than twenty percentage points. However, over the summer, the incumbent appeared to be mounting a comeback. In August, he had closed the gap to single digits. But, by September, the momentum had stopped and Casey again led by double digits.

Santorum is trying to overcome a basic structural challenge in the state--50% of the Keystone State’s voters would select a Democrat if they thought control of the Senate hinged on their vote. Just 40% would vote for GOP control. The power of incumbency can often be counted upon to overcome such numbers, but Santorum is not a typical incumbent and 2006 is not a typical election year.

It's not only unaffiliated voters and Democrats who are sour on Santorum. Casey attracts 18% of GOP voters 27% of conservatives. These numbers may reflect the lingering impact of Santorum's support for Republican Senator Arlen Specter over the more conservative Pat Toomey in Pennsylvania's 2004 Senate primary. Specter was only narrowly re-nominated.

Crosstabs are available for Premium Members only.