Arlen Specter's relationship with the right-wing of his party never been good. But he's managed to keep the conservatives sufficiently at bay to win five Republican Senate primaries. But as the Republican Party becomes more conservative and retreats from the Northeast, his position became untenable. Even if he had been able to move right to fend off his second consecutive primary challenge from conservative Pat Toomey, he would have been extremely vulnerable to a Democratic candidate in the general election. So his decision to switch parties is not terribly surprising.
The big question today of course is the extent to which Specter's switch will affect the success of President Obama's legislative agenda. Of course, as many have already pointed out, the switch plus the probable seating of Al Franken would bring the Senate Democrats to the magic filibuster-proof majority of sixty.
But because there is no guarantee of Democratic unanimity on many of the more controversial aspects of the president's agenda, the effect of the switch ultimately depends on how much Specter moves to the left as he tries to position himself within his new party. Keith, Howard, and I once published a paper that among other things estimated how much party switchers shifted on the liberal-conservative continuum. We found that on average party switchers moved 28 percentile ranks on a liberalism scale. Thus, a Democrat at the 40th percentile on liberalism would move to the 68th percentile. In the 110th Congress, Specter was the 55th most liberal member of the Senate. With the addition of new Democratic senators, he is probably the 62nd most liberal. Consequently, if he shifts the average amount, he'll be the 34th most liberal. Such a move would put him solidly within the Democratic fold near Herb Kohl and Diane Feinstein. He would probably rank more liberal than his fellow Pennsylvania Democrat Bob Casey (even on issues other than abortion).
That's just the average effect. There are reasons to suspect that Specter might go even further left. After all, he has to make party activists forget that he voted for the Iraq War, supported Bush's judicial nominees, and was Anita Hill chief inquisitor (memories are long for these sorts of things). He has to avoid a serious Democratic primary challenge, and he has to raise a lot of money from groups and individuals who have pumped millions into past attempts to defeat him. So Obama just doesn't get an extra vote for his agenda, he gets an easy vote.