Just as Ohio and Missouri are the top two Republican Senate seats I would predict could flip, Pennsylvania is a top seat held by Democrats I could easily see fall the other way*.
So when Quinnipiac says Joe Sestak tied Pat Toomey again, I’m not surprised a second.
Democrats were always going to come home to Sestak in reasonable numbers, but as long as independents favor Toomey, as Quinnipiac says they do 44-35 (MoE 2.7), the situation is good for the Republican. After all, the state that has last voted Democrat for President, Governor, and twice for Senator is not supposed to yield a 43-43 tie for Senate, is it?
* If not for the disaster in North Dakota, Pennsylvania might be the top seat at risk for them.
House of Representatives Swingometer
Electoral College Swingometer
And I’m finding all of Quinnipiac’s 2010 numbers seem to be skewing Dem, so I think the picture is even better than the one you paint. :)