"I am nothing, but I may be everything," John Adams, the first vice president, wrote of his office. And for most of American history, the "nothing" part of Adams's formulation accurately captured the importance of the vice presidency, at least as long as the president had a heartbeat. But a job that once was "not worth a bucket of warm spit," according to John Nance Garner, became, in the hands of the most recent vice presidents, critical to the governing...