Silver: Specter Will Shift Left, But Not Too Far

Now that Arlen Specter is flying the Democratic colors, his new party can expect a tangible—if modest—change in his voting behavior, writes Nate Silver of FiveThirtyEight.com. "Democrats have ample reason to be pleased," writes Silver, who crunched the numbers of all previous congressional party-switchers. "Nevertheless, odds are that Specter will line up squarely in the conservative half of the Democratic caucus and will probably leave room to his left for a primary challenge."

Silver examined the voting changes of all 20 congressmen who have switched parties since 1980. "All of the party-switchers moved toward the direction of their (new) party caucus after making the change, although with somewhat varying degrees of magnitude." Specter's a particularly tough read because he acknowledged that pure politics played a big role. "On the other hand, the parties are now more polarized than they once were, and so crossing the aisle may mean more than it once did."