Because he endorsed McCain, or because he's actually a Republican?

WaPo: [Lieberman's] "Reuniting Our Country" PAC donated $5,000 to McCain's campaign in December....

Lieberman has at least been monogamous on the presidential front this year, but it will be interesting to see how hard he will campaign for McCain if the Arizonan ends up as the GOP nominee. With their two-vote margin in the Senate, Democrats can't afford to have Lieberman cross all the way over and caucus with Republicans. So they'll probably have to bite their tongues while he -- and his checkbook -- try to keep the White House (and a few Senate seats) in GOP hands.