Pennsylvania Sen. Arlen Specter released a statement yesterday announcing that his Senate run next year will be in the Democratic primary—and by the afternoon was sitting on the Democratic side of the aisle. Specter was polling well behind Republican primary opponent Pat Toomey and admitted that he his best chance to stay in the Senate would come by running as a Democrat. Here's a round-up of news and insights from the past 24 hours:

THE PAST
Though Specter has a history of switching parties (as a registered Democrat, he won elected office on the Republican ticket in 1965), he condemned former Vermont Sen. Jim Jeffords' similar party switch in 2001. But his tune changed this year after 200,000 Republicans in Pennsylvania changed registration, leaving the party with largely conservative voters unfriendly to the moderate Specter. (Some say Dems should thank Hillary Clinton for staying in the presidential primary so long, since that helped boost those crossover numbers.) Specter has joked about the switch in the past, and Dems have lobbied hard: Vice President Joe Biden contacted the Senator fourteen times in the past 100 days.

THE FUTURE
Democrats, of course, were pleased—Obama said he was "thrilled" to have the new senator. And party leaders are working hard to clear the primary, though the previous front-runner for the nomination has said he will not lay down. And more liberal Dems think such a challenge is a good thing. Senate Majority Leader Harry Reid had a muted reaction that emphasized the need for bipartisanship. The Republican reaction, of course, was more mixed. Nominal party leader and RNC chair Michael Steele wins for the most colorful reaction, suggesting "his mama didn't raise him this way." The NRCC quickly moved to use the switch as fundraising fodder.

THE FAULT

While some Republicans labelled Specter a hack who was always left of their party, the move added to the growing debate over the space left for moderates in the GOP. Some moderates are blaming the conservative wing of the party for the loss, and Maine Sen. Olympia Snowe laments in the NYTimes that the party "didn't have to lose Specter."

THE VOTE
The big question, of course, is how he will vote: If (or more likely when) Minnesota Democrat Al Franken is seated, Democrats will have a fillibuster-proof 60-vote majority, though Specter has promised not to just be a party-line voter. Labor leaders are nevertheless wooing Specter, who voted for the Employee Free Choice Act in 2007 but said he would not repeat that vote this year. Nate Silver takes a look at the historical precedent of party-switchers to guess that a Democratic Specter will vote along the lines of a Blanche Lincoln or Kent Conrad.