Everyone needs to keep in mind that Specter is significantly to the right of Ben Nelson. This is way, way beyond even Lieberman. Replacing him even with a New Democrat like Sestak would create roughly a 50% shift in voting habits, according to Progressive Punch. No other Democratic primary in the entire country has the same potential for offering even close to such a large shift to the left in voting habits.
Update: Specter just said that he won't invoke cloture on Employee Free Choice. So much for the theory that he would vote against the bill, but allow cloture.