There's yer experience for ya... and Biden has a lot of working-class "authenticity" too, which will help to undermine claims of elitism. It also makes it clearer that Obama is willing to reach across party divides, and will try to solicit advice from even those who occasionally disagree with him. Supporting Biden is also a good way to reach out to most Hillary supporters, even if some PUMAs are beyond help.
Biden's power base is in Delaware -- which should help Obama secure New Hampshire -- and was born in Pennsylvania... which should secure that state, and may help put Obama over in Michigan, while putting Ohio in play.
In other words, Biden does a *very* good job of solidifying Barack Obama in those places which John Kerry won... while Biden's older, Catholic, working-class, pro-veteran, son about-to-be-deployed background should help Obama in several other states across the country... and he's a great, straight-shooting attack dog, who can seriously knock McCain for a loop in situations where Obama might tend to be more reserved.
So, the question, really, is "what state(s) does Obama need to win above that?"
The answer appears to be either one of the following states:
Ohio, Florida, Virginia, Indiana, North Carolina, or Georgia
Two to three of the following states:
Colorado, Iowa, Nevada, New Mexico, North Dakota, Montana, or Alaska.
So, where does that leave McCain?
Most likely, selecting one of the following three candidates:
Mitt Romney, who could bring McCain more support in Colorado, Nevada and New Mexico. In some circumstances, he *could* potentially put Oregon into play for the Republicans, but it would still be an unlikely victory.
In contrast, Lieberman might help with New Hampshire and might counteract some of Biden's influence with the Catholic vote, but at this point, I don't think he can even count on winning in his home state. Picking Lieberman could also cause a divide for McCain with pro-lifers, southern voters, and the more rigid conservatives, essentially demotivating his core constituency.
As for Huckabee, well... it would strengthen McCain in Florida, Virginia, West Virginia, Iowa, and North Carolina... but if McCain can't count on all of those states anyway, he's toast. Likewise, if McCain can't secure Colorado and Nevada, he's toast. That said, Huckabee has the greatest degree of grassroots support, and can potentially reach out to religious voters.
McCain *should* probably pick Huckabee. That said, I suspect he will pick Romney... and, knowing him, do a speech very close to Obama's big night from someplace relatively close, trying to steal Obama's thunder.
Ultimately though, McCain has his work cut out for him. If he can't reverse the current Obama lead in Virginia, Iowa, and Colorado, he loses. So, from a running-mate standpoint, the question for him becomes, "how do I protect myself on all fronts?"
Obama played it pretty safe today, when he possibly didn't have to. Unfortunately for McCain, I suspect he's got to play it even safer if he wants a shot at winning. So, even though he gets the advantage of choosing last, I don't think it's really all that much of an advantage.
(So... can we stop all the speculating yet?!)