Clark helps seal the deal for Obama by calming concerns about his inexperience, yet he doesn't undermine a change campaign the way someone like Bayh or Biden would. I don't get the Kaine boomlet, at all - 3rd best choice from Viginia, no longer even popular there, doesn't bring any valuable experience. The potential shocker: Schweitzer.
McCain - Kay Bailey Hutchinson
He needs a game changer, yet someone who won't piss off the conservatives. He gets to claim history and gives himself a theoretical chance to mitigate the gender gap.
I don't get Romney - it's too late for pre-convention money and the evangelicals won't like it. Jindal is too young, and McCain needs racist votes. The boring, safe choices would Graham, Sanford, or Pawlenty.
I think Clark would be a very good pick - way better than Kaine or Bayh. He provides some balance with his experience and knowledge of international issues and military affairs, yet, as a non elected official he can credibly reinforce the Obama's message of change. He's kind of a Cheney pick (don't get snarky - you know what I mean!)that assures some voters that Obama will pick people that will help him govern.
Of course, Cheney drove the country into a ditch, but I would expect a better result in this case.
In an ideal universe, I think the best choice would be Mark Warner. Of course, he can't be the pick now as picking up that Senate seat is too important. Lot sof others have viewed Webb as another solid choice (I wasn't as sure, but he pulled himself from consideration, anyway).
I can't quite shake the feeling that Kaine looks like the 3rd best choice fromt the state, and whether or nor he's truly the best fit, he'll be portrayed as a regional reach..
There really isn't a decision here from the Dems point of view. He paint him as a conservative and an extension of the Bush years. Bush and the Republicans aren't popular with the public right now, and there's more to be gained by revealing his conservative record (lots of indies think he's pro-choice).
Let the movement conservatives knife him in the back on their own.
Balance is fine, but it should be subordinate to the overall message the top of the ticket wants to communicate, and not contradict it. Richardson, for example, makes some sense for Clinton because he emphasizes her key message (ready from day 1) and also provides some regional and demographical balance. That's fine.
I love Edwards (I voted for him in 2004 and would have this year had he stayed in the race), but he made no sense as VP for Kerry. All he did was emphasize Kerry's liabilities (NE liberal who can't communicate to the common person). It didn't work. Just like putting Bentsen on the ticket didn't convince anyone that Dukakis wasn't a NE liberal or that choosing tax cut enthusiast Kemp made Bob Dole look like he was enthusiastic about anything other than a nap.
If HRC wins the nomination, what does Obama do for the ticket? Is he an experienced executive? We're going to win Illinois one way or the other, right? More importantly, he's a "vision candidate" and putting him on the ticket makes it look like HRC isn't confident in her competing vision for the presidency.
If Obama wins the nomination, again, why would he need HRC? He's emphasizing bringing change to Washington and getting beyond the old squabbles. Putting a Clinton on the ticket does not emphasize change or (fairly or not) certainly doesn't allow us to skip the squabbles of the past.
Balance can work in a VP choice but not at the expense of muddling the nominee's key message.
As I wrote in a comment higher up, there won't be any need to unify the party. The Dems will vote for the nominee. If they are paired, the only thing the media will write about is the "shotgun wedding" aspect. Whoever wins should get to pick the VP that makes the most sense for him or her.