If Obama picks Biden for VP, I think the republicans have this. If McCain picks Romney... then ditto. If the RNC were smart, they'd pick some ultra conservative to point to his succession.
I don't see Obama getting help from any of those in the pool unless it's Hillary, but I think both would rather die than share a ticket. I would imagine that he'd be better served with a more moderate veep than those being touted, as I don't think running a partisan leftist for President with a more established partisan leftist will do much to help his campaign. I'd laugh my ass off if it turns out that former yet now non-Democrat Joe Lieberman gets the nod...it'd widen the ticket's appeal by a light year to cut into the moderate vote that McCain's angling for, and it'd give a big middle-finger to establishment party politics.
McCain OTOH would benefit from a conservative to shore up the base, but rumors keep roiling over him picking a pro-choice moderate. Romney would be better than Tom Ridge, particularly due to his economic acumen. But he and McCain share similar weaknesses in that they're known for rolling on issues and having a light social record. And adding a cult member to the ticket won't suit us fundies who already aren't solidly on the McCain bandwagon. I'm thinking Huckabee's looking really good to them right now, as are several of the other Republican governors like Pawlenty and Palin (the female appeal of whom might help sway Hillary supporters as well as middle-aged male chauvinist pigs).
It boils down to this irrefutable fact: this election will be an up-or-down referendum on Obama. He might as well put out "I Believe in Barry Obama" signs, because it's going to come down to whether people like and trust him or not. Either you foolishly believe in the "change Change CHANGE" mantra and that he'll stop the business-as-usual, Washingtonian, institutionalized politics of the present, or you're scared of him turning us red and decide to vote for the perceived lesser evil. The vice-presidential nominee is immaterial at this point.