The American vice presidency, as the saying goes, "is not worth a bucket of warm spit." Yet vice presidential candidates, many people believe, can make all the difference in winning--or losing--a presidential election. Is that true, though? Did Sarah Palin, for example, sink John McCain's campaign in 2008? Did Joe Biden help Barack Obama win? Do running mates actually matter? In the first book to put this question to a rigorous test, Christopher...