The Washington Post confirms:
Pennsylvania Sen. Arlen Specter will switch his party affiliation from Republican to Democrat, according to sources informed on the decision.
Specter's decision would give Democrats a 60-seat, filibuster-proof majority in the Senate assuming Democrat Al Franken is eventually sworn in as the next Senator from Minnesota. (Former Sen. Norm Coleman is appealing Franken's victory in the state Supreme Court.)
Specter as a Democrat would also fundamentally alter the 2010 calculus in Pennsylvania as he was expected to face a difficult primary challenge next year from former Rep. Pat Toomey. The only announced Democrat in the race is former National Constitution Center head Joe Torsella although several other candidates are looking at the race.
The number 60 doesn't assure 60 votes on any particular priority, but it's a tremendous psychological advantage. It means that there are 60 votes with a baseline incentive to see a successful Democratic Party rather than an unsuccessful Democratic Party. There are 60 senators, in other words, who are free from the incentive structure of the minority. That doesn't guarantee legislative success, of course. The last time the majority party controlled both the executive branch and 60 seats in the Senate was 1975 to 1979, which is to say, under Carter, and that didn't work so well. But the Dixiecrats are gone, the committee chairmen are less powerful, and the relationship between the executive and the Congress is substantially less adversarial. This has the potential to really reshape the playing field.
One more thought: Like with Southern Democrats switching to become conservative Republicans, Northern Republicans often prove to be fairly liberal Democrats. Jim Jeffords is a good example of this. So once Specter actually comes on-board, he may be a much more reliable vote than someone like, say, Ben Nelson. And as a new Democrat in a blue state, Specter will be very safer if the Democratic base appreciates him than if they mistrust him.
Update: Specter's statement is here. Key quote:
Since my election in 1980, as part of the Reagan Big Tent, the Republican Party has moved far to the right. Last year, more than 200,000 Republicans in Pennsylvania changed their registration to become Democrats. I now find my political philosophy more in line with Democrats than Republicans.
Baseless speculation: Did Specter's decision to oppose the Employee Free Choice Act force his switch? He was sort of caught between a rock and a hard place. Pat Toomey was going to primary him on the right. In past years, that was fine, as he had tacit support from elements of the left, namely the unions. But since betraying them on EFCA, the unions were promising to make Specter their main target. So if he stayed in the Republican Party he would lose the base at the exact moment that Republicans had lost their capacity to talk to independents and that the most powerful of lefty interest groups was gearing up for a massive campaign against him. He was a many with no home. By joining the Democratic Party, he could effectively neuter the unions while gaining the whole of Pennsylvania's Democratic Party machine. But he only really needed to do that because of EFCA.