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." (More Arlen Specter stories.)