From the WSJ:
Sometimes the vice-presidential decision results from the campaign's flow. In 2008, John McCain's camp felt that while they were ahead after a better-than-expected summer, they needed to shake things up with an out-of-the-box pick. This thinking produced Sarah Palin.... But such political decisions run into one hard reality: Running mates haven't decided an election in more than a half-century. For example, research by Bernard Grofman and Reuben Kline, political scientists at the University of California, Irvine, suggests that the net impact of the vice-presidential picks in 2008 was roughly one-half of one point and is generally less than one percentage point. Presidential elections are rarely that close.

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