My advice for Obama would be to pick someone who's a great advocate, regardless of how liberal or moderate they are.
Right now, in about 80% (at least) of the controversial cases that fall along the expected lines, Roberts is persuading Kennedy his way. He's just killing the more liberal justices on this front, and somehow managing to temper Scalia's excesses at the same time.
Another vote isn't going to change that (and an ideologue could make things worse), but an advocate could have some effect.
Sam's right, but I'm not sure how much that narrows it down.
