McCain with Ridge, near right, and Pawlenty, far right. AP Photo/Charles Dharapak

Barack Obama and John McCain disagree about plenty of things. Iraq. Taxes. The best time to vacation in Berlin. Now, judging by the available evidence, we have to add yet another rift to the list: how they plan to pick their vice presidents.

Asked about internal matters, Obama's staff is famously tight-lipped. So it came as something of a surprise on June 25 when campaign manager David Plouffe told a gaggle of D.C. reporters something--anything--about what the Illinois senator is looking for in a running mate. Obama will select someone "qualified to be president and someone who'll be a partner in governing," said Plouffe. Geographical convenience--as in, will he or she help him swing a state?--won't come into play.

It's one of the oldest questions in the veep-vetting book: can a running mate really deliver his home state? By disregarding what many experts consider a key VP criteria, Obama was signaling his electoral confidence. I'm doing well enough, he seemed to say, to base my choice on "principle," not pragmatism. McCain, however, doesn't have the same luxury. In fact, looking at his shortlist--former Massachusetts governor Mitt Romney, Minnesota Gov. Tim Pawlenty, former U.S. budget director Rob Portman and former Pennsylvania governor Tom Ridge, according to today's Washington Post--it's clear that only one thing unites the contenders: the fact that none of them would be in contention for anything if they hailed from, say, Idaho.

Take Romney, for example. His business acumen is impressive. He's an able and willing attack dog. But it's no secret that McCain--who, for politician, places an unusual premium on personal relationships when deciding who to trust--destested him during the primaries. Not to mention that his flip-flops on social issues have alienated both Christian conservatives and crossover voters. So why has Romney's stock steadily risen inside the campaign--and why has McCain gushed about him on the trail? Because (as we've noted before) he's a proven vote-getter in Michigan who could possibly swing the state's 17 electoral votes into the Republican column for the first time since 1988. Now, according to Bob Novak's upcoming column, McCain's internal polling agrees. "Private polls show Romney could make all the difference in Michigan," he writes. "A McCain-Romney ticket carries the state by a moderately comfortable margin. With any other running mate, McCain loses Michigan.” Voila.

Romney at least seems like a credible president. Portman and Pawlenty? Not quite. McCain has said that he's "aware of the increased importance" of his veep pick "given [his] age" and will therefore select "someone who can take [his] place immediately." But even though Portman and Pawlenty have their charms--Portman's an economic whiz who ran the OMB under President Bush; Pawlenty's blue-collar persona appeals to right-wingers and centrists alike--neither would strike McCain as ready for the Oval Office if he were, say, running in the Republican primaries. Portman, 55, has never been elected to executive office; Pawlenty, 47, has only served one full term as governor. It's clear that Minnesota (where McCain trails by two in the latest poll) and Ohio (where he's behind by less, on average) are their most persuasive attributes. Same goes for Ridge. Unlike Portman and Pawlenty, the former congressman, governor and Homeland Security director is undoubtedly ready for the gig; unlike Romney, he's close to McCain. Problem is, he's also a pro-choice moderate. Given McCain's existing issues with the religious right--a large segment of the GOP base--the prospect of flipping Pennsylvania, where he currently lags by eight, is probably the only thing keeping Ridge in the mix.

Is McCain foolish to rely so heavily on geographical convenience? Historians would say yes. As every dust-caked, tweed-clad presidential scholar loves to point out, no veep pick since Lyndon B. Johnson in 1964 1960 single-handedly swung a decisive state for his boss. But we happen to think McCain is on the right track. For starters, the only reason LBJ was the last VP to swing a decisive state is because JFK was the last nominee to select his partner with that express purpose in mind. In 1968, Nixon suspected he would lose Maryland, despite Agnew's presence on the ticket, and while Muskie helped Humphrey win Maine, it wasn't nearly enough to win the election. In 1976, Mondale's home state of Minnesota was already leaning blue; Dole's home state of Kansas was already solidly red. Dukakis didn't expect Lloyd Bentsen to deliver Texas. John Edwards was hardly Kerry's best bet for a map changer. A son of the South, Clinton would've won Tennessee with or without Gore. And Geraldine Ferraro, Dan Quayle, Dick Cheney and Joe Lieberman didn't even come from swing states. Which is why it can't hurt for McCain to make the effort.

The truth is, the Arizona senator will not win in November without Ohio--and winning will be very difficult without either Michigan, Minnesota or Pennsylvania as well. The math is simple. Obama is currently leading in every state that Kerry won in 2004. They're worth a total of 251 electoral votes. In addition, he also holds slim advantages in the Bush states of Ohio, Virginia, Colorado, Iowa, New Mexico, Nevada, Indiana and Montana. Of those 74 electoral votes, Obama needs a mere 19--Iowa, New Mexico and Colorado, for example--to claim victory. The only way McCain can compete is by either a) holding his rival to fewer pickups or b) picking up some electoral votes of his own. Ohio would help accomplish former. Michigan, Minnesota or Pennsylvania would help accomplish the latter. Ultimately, putting a Ridge, a Romney, a Portman or a Pawlenty on the ticket won't guarantee a McCain victory on Election Day. But it may be his most realistic shot.