In a closed primary, Joe Lieberman's support of McCain would probably not mean much. But in Michigan, which remember is open, it could make the difference. From ABC News:

"I urge [Democrats and Independents] to come out today and stand for a man who is a great American patriot," Lieberman said at a joint town hall meeting with McCain in Ann Arbor, Mich.

"There's no competitive Democratic primary," Lieberman added. "I know there's some other moderate and conservative Democrats like myself."

Then again, perhaps Democratic voters will heed Daily Kos' call to vote for Mitt Romney. At least we'll know in a few hours.