Now that he's virtually sewn up the Democratic nomination, John Kerry has started his search for a running mate. One rumor making the rounds is that Kerry will make entreaties to Sen. John McCain, a Republican. Is it really possible for a presidential candidate to select a running mate from a different party?
Not only is it theoretically possible, it's actually happened, albeit under very unusual circumstances during the Civil War. There is nothing in either party's convention rules that specifically prohibits a candidate from selecting a running mate from the opposition. In practice, of course, it's extremely unlikely that a Republican would consent to run on a Democratic ticket, or vice versa. The traitor would certainly become persona non grata among their old partisan allies, to the point where they'd probably be forced to switch party affiliations anyway. The odds are slim to none that McCain would be willing to invite such scorn from his political chums.
It's also possible that a party's rank-and-file would coordinate a revolt on the convention floor and, going against tradition, work to subvert the presidential nominee's choice. According to the rules of the Democratic National Committee, for example, a vice presidential nomination is supposed to be affirmed by a roll-call ballot, a process that is typically a mere formality. But the selection of a Republican running mate could inspire many delegates to oppose the ticket's lesser half during the balloting. As the DNC rules note, "Delegates may vote for the candidate of their choice whether or not the name of such candidate was placed in nomination." At the very least, a ruckus like that would leave a bitter taste in everyone's mouth, at precisely the worst moment possible.
Neither party seems to put much stock in the McCain rumor, nor in whispers that President Bush could dump Vice President Dick Cheney in favor of conservative Democrat Zell Miller. As a flummoxed Republican National Committee staffer told Slate when asked, "I don't think it's written down anywhere that you can't [nominate a Democrat], but I can't believe they'd let it happen."
But it did happen once, in 1864. Republican incumbent Abraham Lincoln selected a Democrat, Andrew Johnson, as his running mate. That was during an extraordinary time, of course, and it should be noted that the pair didn't technically run under the GOP banner. Rather, they ran on the ticket of the National Union Party, a wartime coalition between Republicans and Democrats who (like Johnson) were against secession.
Prior to that, another mishmash occurred in 1796, when the Federalist John Adams won the presidential election and the Republican Thomas Jefferson was his vice president. But that was back when members of the Electoral College cast two votes for president, and the runner-up got the vice president's job. The 12th Amendment fixed that in 1804, mandating that members of the Electoral College cast one vote for president and a separate one for vice president.
Bonus Explainer: There's another far-fetched scenario in which the president and vice president could come from opposing parties. In the event of a tie in the Electoral College, the election is thrown to Congress—the House picks the president, while the Senate picks the vice president. It's entirely possible that one could select a Democrat while the other chose a Republican.