10. Because he's the most Vice-Presidential candidate we have. For VP, you want a powerful campaigner who can use the media spotlight associated with the position to promote progressive ends and support the presidential candidate's major initiatives. Edwards is a good campaigner and speaker who knows how to focus media attention into places where it needs to be (see: poverty and health care). I'd contrast him with someone like Bob Graham, recently spotlighted in this space, who's much more the kind of guy you want in the Cabinet -- a quietly effective smart old guy with few campaigning skills and non-camera-friendly tendencies.
9. Because economic issues are huge this year. Matthew Yglesias presents this chart in a post titled "Annals of GOP doom":
This isn't 2002, 2004, or 2006. Economic worries are bigger this year than they've been for the last 30 years. In better times, I'd be more sympathetic to Matt Stoller's case for Wes Clark, but this is time to capitalize completely on the issue that voters think is the most important by substantial margins -- the economy. Obama's Iraq foresight will serve us well on foreign policy, and we need somebody who can make working-class voters in the economically depressed Midwest see McCain's anti-worker record and vote Democratic. Nobody in our Two Americas does it better than John Edwards.
8. Because conciliating Clinton supporters with a Clintonite is a fool's errand.By historical standards, this actually hasn't been a particularly divisive primary -- only 1/4 of Clinton supporters said they wouldn't vote for Obama. By contrast, a full 51% of McCain supporters said they wouldn't vote for Bush in March 2000. They still voted for him enough to give him the election. Nobody means what they say when you ask them that question at the most emotional point in the process. It's one of the reasons why I don't get angry at Hillary supporters who say they'll sit this one out -- most of them will rethink that in cooler moments and make the right decision. So don't do outlandish things for party unity. You'll get it anyway.
And unless my reading of the psychology is totally confused, Hillary's core supporters are attached to Hillary herself, who is in many ways an inspiring and sympathetic figure. Ed Rendell, Evan Bayh, and even Wes Clark aren't what their dreams are made of. So even if you think we won't get party unity for some reason, please don't think you can get them to act like Hillary's on the ticket just by picking some dude who endorsed her. (As for picking Hillary herself, I lay out a bunch of reasons not to do that here.)
7. Because running with someone who repents his pro-war vote throws Obama's skills into full relief, and keeps the focus on 2002. There's no way Obama should pick an unrepentant Iraq War supporter like Clinton -- he needs a fellow war opponent to make broad and effective criticisms of the neocons who started the wa. As long as Obama has someone who wholeheartedly supports his early antiwar position and is eager to praise his foresight, he'll be in good shape.
Republicans have a superfically plausible case to make about the surge, even if it in fact neglects the entire purpose of the surge (to make room for political reconciliation). By contrast, the 2002 Iraq vote is nearly impossible to defend, and disapproval of the original invasion is around 70% in most polls. As Justin Tiehen suggests, media coverage of the relationship between Obama, who foresaw all the dangers at the beginning, and Edwards, who now hopes to redeem his acknowledged error by going all-out in favor of his prescient ticketmate helps to keep focus in the place where the Republican position is obviously indefensible. (There's also a really nice regional / racial redemption story in here, if you're looking for it.)
6. Because Pennsylvania tells us that Edwards helps Obama more than a locally popular Democrat. Take a look at the polling data:
How much does the Governor of Pennsylvania help us in his home state? Not as much as John Edwards does. Edwards is 3-5% better than Rendell in the state where Rendell is supposed to help us win. You don't pick John Edwards to help in the South or the Carolinas -- you pick him to help everywhere, because that's what he does. If you want more numbers, please go see OpenLeft's Paul Rosenberg, who concludes: "Edwards is superior to all other VP candidates by margins that persist in virtually every category in almost every state."