Sure, he has been progresssively moving to the Democratic side, but that was due more to his own party's nuttery more than a love for the Democrats. Andrew Sullivan ranked #34 amongst conservatives in the same feature where Markos ranked #12 amongst liberals (here).
This month's Atlantic has his endorsement of Obama, and it can be found online at their website here.
Not only does Sullivan give his reasons for Obama, but he also gives his reasons for wanting no more of Hillary. It is an interesting read and worth checking out.
My favorite (or, since he's British, I guess I should say favourite) section was where he turns what the right would use against him as a stength:
Consider this hypothetical. It’s November 2008. A young Pakistani Muslim is watching television and sees that this man—Barack Hussein Obama—is the new face of America. In one simple image, America’s soft power has been ratcheted up not a notch, but a logarithm. A brown-skinned man whose father was an African, who grew up in Indonesia and Hawaii, who attended a majority-Muslim school as a boy, is now the alleged enemy. If you wanted the crudest but most effective weapon against the demonization of America that fuels Islamist ideology, Obama’s face gets close. It proves them wrong about what America is in ways no words can.
Now let's see if Obama can pick up that idea and run with it.