We shouldn't underestimate the role the VP played in getting Specter to switch parties:
At the White House, Vice President Joe Biden took the lead in trying to woo his former Senate colleague of 28 years.
Biden is the point man for the administration’s outreach to Senate Republicans, and Specter had been a top White House target all year. It was Biden who played the key administration role in delivering Specter’s vote on the stimulus.
More than that, Biden — often called the “third senator from Pennsylvania” — had been friends with Specter since the former Republican was elected to the Senate in 1980. They had shared many an Amtrak train trip together and sat next to one another for a time on the Judiciary Committee.
Biden, a top aide said, had been making the case for Specter to switch for at least five years but had intensified that effort since the stimulus. The conversation became more rich the more trouble Sen. Specter was having in Pennsylvania and the more the Republican Party lurched right,” said the aide.
In the past 10 weeks, Biden has spoken with Specter 14 times — six in person and eight on the phone, a White House official said — though many of those conversations were devoted to policy issues.
Biden made the case that Specter had no place in the increasingly conservative GOP and that the moderate Northeastern Republican was a thing of the past. You’re alone in this party,” a Biden aide said, recounting the vice president’s message. -Politico
This is why Obama picked Biden, and this probably gets Biden amnesty for a year's worth of upcoming gaffes.
