If I were advising Obama, I'd have told him to pick Bill Richardson instead of Biden. No baggage, one of the true good guys in politics, fills all Obama's holes even better than Biden does, continues the nebulous "change" theme, etc.
Back to McCain/Clinton. Fully 1 in 5 Clinton voters are backing McCain, according to the polls. That number would be 80% or more if she were on the ticket. The Dems' problem may be that their own base doesn't turn out in the kind of numbers required to keep the election close or win it.
Back to the math. It's a subjective guess as to how many Republicans would stay home, but if it's 1/2, then the math still is hugely in McCain's favor. 25M + 18M = 43M to Obama's 32M. You start with those numbers and figure out where Obama's going to get more votes and where McCain loses more to make the race close.
It's moot tho.
For the record, I'm writing in Ron Paul, and don't support either Obama or McCain. If I had my way, govt. would be cut by 2/3, and neither candidate comes close to my view of the role of govt. You have Obama with a laundry list of $trillion spending ideas, and McCain has presented no vision of the future that I can see. Obama talks of escalating the war in Afghanistan, which makes absolutely no sense to me, while I at least am satisfied to ride out the situation in Iraq as the end appears near enough.