Very big political news this morning, indeed.
The move makes sense for a few reasons:
1) Specter could not have lost the primary and run as an independent, like Joe Lieberman. State law prohibits from doing so, as this The Hill article makes clear.
2) He was way down in Republican primary polls.
1) Will the Franken-Coleman court challenge become more acrimonious since his seating now means 60, and not just close to 60?
2) Can Specter win the Democratic primary? (Nate Silver asks why Democrats will settle for a "Lieberdem")
In addition, Sens. Snowe and Graham commented on Politico about the Republican Party attitudes towards moderates (as Snowe said, "You're with us or you're against us.")