According to a new poll from Susquehanna Polling and Research, conducted for the Pittsburgh Tribune Review, the Senate seat considered safe for much of this election has now become a toss-up.  Republican challenger Tom Smith has gained four points in a month to come within a point of Democratic incumbent Robert Casey, in a state where Mitt Romney has begun to focus on his final campaign drive:

Republican U.S. Senate nominee Tom Smith narrowed the race against Democratic Sen. Bob Casey Jr., pulling nearly even in a Tribune-Review poll.

Casey polled 46 percent to Smith’s 45 percent among likely voters, with 8 percent undecided, according to the survey by Susquehanna Polling & Research in Harrisburg.

Those figures show stagnation for Casey but a 4-point gain for Smith since a Trib-commissioned poll Sept. 12.

The more recent poll of 800 people from Oct. 29-31 has a margin of error of 3.4 percentage points.

The story doesn’t include the sample, but the partisan split is D+6 at 44/38/18, as I recall from my interview with Jim Lee of Susquehanna.  The last Franklin & Marshall poll, which Casey’s camp cites as a rebuttal to Susquehanna, had a D+13 sample of 50/37/13 and gave Casey an 11-point lead.  Lee feels this kind of split widely oversamples Democrats in relation to turnout, and on the basis of history, he’s obviously right.  The turnout in Pennsylvania in 2008 was 44/37/18, almost identical to what Lee uses in his weighting, and arguably a high-water mark that Democrats aren’t likely to reach four years later.  The difference between the two polls is almost entirely found within that difference in party split.

That brings me to my next point.  The Franklin & Marshall poll that showed Casey 11 points up in a D+13 sample also had Barack Obama only four points ahead of Mitt Romney, 48/44.  If there’s a a ten-point difference between the two polls in the Senate race, what might be happening in the presidential race?  We will find out tomorrow, as Susquehanna and the PTR has the poll results in the presidential race coming tomorrow.  I’d bet we’ll see why Mitt Romney will make one last big push in the Keystone State, and why Obama and his campaign are dumping a fortune into Pennsylvania at the last minute.