Mark Liberman at Language Log has an excellent analysis of politicians' slips of the tongue, focusing on this item from ABC News:

When introducing his running mate, Obama said, "So let me introduce to you the next president - the next vice president of the US of America, Joe Biden."

And then when it was Biden's turn to speak, the Delaware senator called the presumptive Democratic nominee "Barack America" instead of Barack Obama.

"My friends, I don't have to tell you, this election year the choice is clear. One man stands ready to deliver change we desperately need. A man I'm proud to call my friend. A man who will be the next president of the United States, Barack America," Biden said, per ABC News' Sunlen Miller.

Liberman points to particular linguistic reasons why these particular kinds of slips are commonplace, and concludes — rather more categorically and forcefully than I would, but on balance basically soundly — with this:

With respect to speech-production blunders all across the political [spectrum], our perspective has been consistent[]. Everyone commits speech errors, even professional talking heads, and anyone who makes a big deal about particular examples is either a fool or a hypocrite. Since fatigue, stress, and complex ideas all promote speech errors, you can depend on political rhetoric to provide plenty of occasions for foolishness and hypocrisy.