Like bread and butter

The ideal VP would complement the Presidential candidate - be the butter to his bread.

In the Obama camp, rumors are currently flying that Senator Joe Biden of Delaware is Obama's top choice (though Biden denies this). It makes sense: Biden, as the Senate Foreign Relations Committee chairman, could provide the strength and muscle to Obama's foreign-policy Achilles' heel. That's achieving one goal when picking a VP (as Earl noted): making up for what you lack, a.k.a. do what'll reap the greatest benefits at the polls.

On the other hand, McCain is stuck in a controversial situation: he named pro-choice Tom Ridge (former governor of Pennsylvania and ex-head of the Department of Homeland Security) as a strong VP possibility. This leaves McCain in a political pickle - alienate the ultra-conservatives or attract those who are more in the center?