Name: Mitt Romney
Age: 61
Resume: Former governor of Massachusetts, 2008 Republican presidential candidate

Source of speculation: How Romney and McCain spent their Memorial Day weekend: together. On Friday, the presumptive Republican nominee split from the campaign trail for a three-day break at his ranch in Sedona, Arizona. But he didn't put all presidential preparations on hold. Joining McCain for what aides insistently--and euphemistically--described as a "social weekend" were three of his top veep contenders: Gov. Bobby Jindal of Louisiana, Gov. Charlie Crist of Florida and, of course, Romney. What a coincidence--or not. On background, associates admitted to the New York Times last Wednesday that "while these would not be formal interviews, the weekend would provide Mr. McCain a chance to know some of his potential running mates in a social context." Think "Survivor: Veepstakes Edition." Horseshoes, anyone?

Backstory: Since quitting the Republican race on Feb. 7, Romney has made no secret of his desire to serve as McCain's second fiddle. “I think any Republican leader in this country would be honored to be asked to serve as the vice presidential nominee, myself included," he told FOX's Sean Hannity on March 11. "If the nominee of our party asked you to serve with him, anybody would be honored to receive that call … and to accept it, of course.” Meanwhile, two of D.C.'s most wired conservative journalists--Bob Novak and Fred Barnes--have reported that all the president's men are pushing Romney for number two. "President Bush favors him as McCain's veep," wrote Barnes in March. "Jeb Bush, the former Florida governor, preferred Romney over McCain in the primaries, but never endorsed him publicly. Karl Rove, the president's political strategist, has hinted that he considers Romney to be McCain's best running mate." And, lest we forget, Romney has even paid McCain the most flattering of canine compliments: "when it comes to national security, John McCain is the big dog--and [Clinton and Obama] are the chihuahuas" Woof.

Odds: Moderate. As a running mate for McCain, Romney is almost all upside. For starters, he's a decade younger than McCain, 71, and looks about 50--a key consideration for a candidate who often claims he's "older than dirt." As veep, Romney's executive experience and private-sector expertise--before serving as Massachusetts governor, he was CEO of Bain and Company, co-founder of Bain Capital and savior of the troubled 2002 Salt Lake Olympics--would help dispel doubts about the managerial and economic acumen of his partner, a career legislator who often shows little interest in subjects other than foreign policy or government reform. ("The issue of economics is not something I’ve understood as well as I should," McCain has infamously said.) Regarding the electoral map, Mitt, unlike other VP hopefuls, won't help McCain win his home state--the deep-blue Massachusetts--but he could very well boost his boss's bid in the increasingly purple swing state of Michigan, where his father was governor and the Romney brand is strong. With proven skill at soliciting donations and a personal fortune of $250 million--which, we should note, he's not afraid to spend campaigning for office--Romney could also help close the massive fundraising gap between McCain and Barack Obama. And by bringing to the ticket a built-in base that was large enough to keep him neck-and-neck with his former rival from Arizona through Super Tuesday--and consists, with its blend of hard-core fiscal conservatives and family-values voters, of Republicans who've been somewhat wary of embracing the unorthodox GOP nominee--Romney would probably do the most of all the potential picks to excite dispirited, ground-level Republicans. That alone is a substantial bright spot in these dark days of GOP decline and Democratic enthusiasm.

Still, Romney is only "almost" all upside--and his biggest liability is a potential game-ender. It's not so much that Romney ran an extremely confused (and confusing) primary campaign, tacking wildly from "Reaganite rightwinger" to "moderate technocrat" over the course of 2007 and early 2008; although Democrats would surely paint him as a hypocrite, citing his flip-flops on abortion and gay marriage, Mitt's chameleonic talents might actually come in handy during a general election in which wooing Independents is at least as important as appeasing the base. And it's not that Romney spent months attacking McCain in precisely the same way that Obama is now attacking him ("John McCain can't CHANGE Washington," he once said. "John McCain IS Washington"); sure, the Dems would cut a few biting ads, but voters' memories are reliably short when it comes to such shenanigans (after all, John Edwards' 2004 primary season swipes at John Kerry didn't haunt the joint ticket that fall).

In the end, it seems Mitt's major problem isn't political--it's personal. In the wake of Romney's February withdrawal, it was widely reported that Team McCain considered their recently vanquished rival an unprincipled opportunist, and in interviews and speeches at the time, the candidate was barely able to keep his contempt in check. Since then, Romney and McCain have raised money together, and Romney, for one, has said "there are really no hard feelings, I don't think." But an lingering animosity doesn't bode well for Mitt. For a pol, McCain places an unusually high premium on friendships and loyalty--and, according to the Times, "associates [have already] said personal chemistry would be a crucial consideration in his choice" for veep.

That said, McCain had little trouble overcoming his resentment toward President Bush--who sunk McCain in South Carolina in 2000 with a whispering campaign about a "black" love child--and embracing him when it became a political necessity. Who's to say he won't repeat the process with Romney?

Talk about a high-stakes game of horseshoes.

UPDATE, 9:30 p.m.: Commenters brett12 and gawksquawk make some smart points. Here's the latter:

Romney would be a major liability to McCain in the south and mid-west.  I think it would all but guarantee Obama victories in Iowa and Virginia, as well as put North Carolina and Missouri squarely in play. During the Iowa campaign, it almost seemed like the evangelicals were determined to find another candidate so they wouldn't have to vote for McCain or Romney.  First they turned to Fred Thompson.  When it turned out that he didn't want to run for the job they turned to Mike Huckabee.  Huckabee went on to win the majority of evangelical voters until he dropped out of the race.  Evangelicals don't like McCain because they view him as too moderate.  They don't like Romney because he allowed himself to be perceived as pro-choice and pro-gay-rights when it suited him as Governor of Massachusetts. The combination of the two of them on the same ticket may drop evangelical turnout.  When combined with the increased African-American turnout Obama is expected to generate, this may result in the Democrats winning in some surprising places. One upside you didn't mention is a potential boost in Nevada and Colorado the ticket would receive from Mormon voters.  They're not nearly as numerous in those states as they are in Utah, but in a close race they could make the difference.