Marc Ambinder has a nice post on what's running through McCain and Obama's heads right now. I'd add to that that their supporters, who have a tendency to focus on this election, are talking about this much more in terms of who helps win the next election rather than who would be a good stand-in or successor. But the evidence is very weak that vice presidents ever help win elections, and very strong that vice presidents frequently go on to become presidents. Johnson succeeded Kennedy, Nixon (eventually) succeeded Eisenhower, Ford succeeded Nixon, Bush succeeded Reagan, and if there hadn't been vote chicanery in Florida, Gore would have succeeded Clinton. Obviously, you can't toss out political concerns entirely, but they really need to be paired with a discussion of who would actually make a good president.
