Pennsylvania Senator Arlen Specter announced he's switching parties and will now caucus with the Democrats. In 2010, he will run for reelection as a Democrat. Read about it here from the Wall Street Journal, Washington Post or New York Times. Here is a timeline of his career.
What does this mean?
Most obvious is the boost to Senate Democratic fortunes. The Caucus will now have 59 members (soon to be 60 whenever Al Franken gets confirmed to the Senate). This means the Democratic majority is nearing filibuster-proof majority status. Mitch McConnell and Senate Republicans will be virtually devoid of parliamentary tactics to deny the Democratic super-majority.
Less obvious, but more important is why Senator Specter chose to switch. Much less like Vermont Senator Jim Jefford's rejection of the GOP in 2001, Specter is uncomfortable with a party that is far to the right of where it once was.
In his statement, Specter notes, "the Republican Party has moved far to the right." In reality, he joins the 200,000 Pennsylvanians that also switched from registered Republican to registered Democrats during the 2008 election campaign.
This is yet more evidence of how the GOP is an endangered species in the northeast and New England. The Party is geographically isolated because it is ideologically narrow. Until the party broadens its' idea base, its' voter base will continue to shrink.