Kate Sheppard adduces some pretty good reasons to think that putting Condoleezza Rice on the ticket could be a smart move for John McCain. I think there's a lot of truth to what she says, but at the end of the day one needs to return to the fundamentals. It'd be hard for the incumbent party to hold onto the White House amidst serious economic problems and an unpopular war. McCain's viability as a candidate rests on him not being seen as four more years of Bush. That means you don't want a Bush cabinet official on your ticket, and certainly not a Bush cabinet official well-known for her close personal relationship with the President.
I have a similar reaction to Marc Ambinder's suggestion of Joe Biden for Barack Obama. Biden's a sometimes maddening figure, but he's been impressive lately and there's a lot to be said on his behalf. But putting someone who voted for the war, even someone who did so half-heartedly and after making a quasi-promising effort to restrain Bush, seems to muddy way too much of the argument Obama is making.