From Ohio to Louisiana, in swing states and the heartland, John McCain and Barack Obama are turning their attention to the next big decision of the 2008 election cycle: choosing a running mate. Both men have needs to fill. McCain's weaknesses call for a younger leader who can shore up conservative support while representing a new direction for the GOP (and appears ready to be President if necessary, given McCain's age). Obama must build a bridge to centrist white voters while bolstering his foreign policy credentials and improving his support among women, many of whom think he should simply pick Hillary Clinton as his VP. Here's a look at some of each presumptive nominee's top contenders, as well as some dark horses.

By Massimo Calabresi, with reporting by James Downie/Washington