My couple of observations of reality for Sestak supporters:
1) Momentum (I'm a little concerned that Sestak is peaking too early, fwiw)
2) Sealing the deal (A certain bit of the big movement will stick, more likely than not it would have happened anyway, but the polling says that Sestak's "made the offer". Over the next few days, we'll know whether he can close the deal. Recently, think of this as similar to what happened in Britain.)
The problem I can't get around is that these two candidates were not created equal. Specter is both the incumbent and one who was an opponent of the Democrats for 30+ years. Sestak is doctrinally sound and not well-known enough to have earned real enemies. Specter is relying on the votes of hundreds of thousands of people who voted against him in 2004. It's hard for me to see how he pulls it out. Speaking as a Democrat, there are precious few ways I could imagine Specter having "sealed the deal" with me or other Democrats when the only rationale, his comparative electability, has been seriously called into question.