Senator McCain, who is 71, could blunt questions about his age by picking someone younger but with the clear qualifications to take his place, analysts say. For Senator Obama, a first-term senator who is closing in on the Democratic nomination, the choice of a seasoned statesman could mute criticism that he lacks the experience to be president.
"On Obama's side, you have a problem somewhat similar to what President Bush had seven years ago – trying to reassure people that you'll be surrounded by formidable resumes and formidable experience in government," says Joel Goldstein, a professor at St. Louis University School of Law and an expert on the vice presidency. "On McCain's side, you have the need for a plausible successor." Vice presidential picks are typically announced close to the national party conventions in late summer. But with the primary season now winding down, talk of the No. 2 slot is heating up in both parties.
McCain, the presumptive GOP nominee, barbecued for three candidates at his Sedona, Ariz., ranch over the Memorial Day weekend: former Massachusetts Gov. Mitt Romney, Florida Gov. Charlie Crist, and Louisiana Gov. Bobby Jindal.
Last Thursday, word broke that Obama had tapped Jim Johnson, the former Fannie Mae CEO, to lead a team vetting a long list of candidates. The choice itself is telling: Mr. Johnson is a consummate Washington insider who played a similar role for Sen. John Kerry in 2004 and Walter Mondale in 1984.
Names mentioned for the Obama ticket range from Sen. Joseph Biden Jr. and former Georgia Sen. Sam Nunn – both foreign-policy sages – to governors like Kathleen Sebelius of Kansas, Janet Napolitano of Arizona, and Tim Kaine of Virginia. Also on some lists are Sen. James Webb of Virginia, a Marine and former Republican and Navy secretary, and Sen. Claire McCaskill of Missouri, an early Obama backer from a key swing state.
Some party leaders – including, reportedly, Bill Clinton – are pressing for a so-called "dream ticket" with Hillary Rodham Clinton to unify the party. But several analysts say they see an Obama-Clinton pairing as unlikely, both because of the acrimony of the long nomination fight and because of the awkwardness of having an ex-president as the vice president's spouse.
"How would you tell a former president of the United States to shut up and toe the party line," says Dr. Goldstein. "It would be hard to take the microphone away from him."
McCain has said he wants a vice president who could "immediately take my place.
"The sole criteria I'm looking at … is who can best take my place and carry on the agenda and the vision that I have outlined and will continue to outline during this campaign," he said on Fox News Sunday last month.
Obama has been more reticent, mostly because he is still about 50 delegates away from formally sealing the Democratic nomination. But in response to an audience question in Boca Raton, Fla., last week, Obama signaled that he saw the Abraham Lincoln White House, which was stocked with political rivals, as a model and said he wouldn't rule out a Republican No. 2.
"My goal is to have the best possible government, and that means me winning," he said. "So, I am very practical-minded."
Analysts say he is almost certain to pick a veteran political figure, probably with a foreign affairs portfolio. "He really needs somebody with gray hair," says Michael Nelson, a political scientist at Rhodes College in Memphis, Tenn., and author of "A Heartbeat Away," about the vice presidency. "Not only gray hair, but somebody with long and deep and visible credentials in foreign policy. That almost dictates a Washington choice."
McCain's deliberations are likely to be more complicated. He will want an experienced leader seen as ready to step into his shoes, but someone new enough to generate excitement. If McCain chooses another older white man also seen as a Washington fixture, it could hurt GOP chances in a year when voters of every stripe are calling for change.
Analysts view Mr. Romney as a favorite. He is a former governor and private-sector executive whose domestic policy and business background complement McCain's record as a legislator and foreign policymaker. He is also a social conservative, which could draw in right-wing Republicans wary of McCain's ideological bona fides.
Finally, Romney is a prodigious fundraiser in a year in which Democratic candidates have vastly outraised Republican ones.
McCain associates have said the invitations to his Arizona ranch this past weekend were "purely social." But more likely, analysts say, McCain and his aides were judging the candidates on an oft-overlooked aspect of vice presidential searches: personal compatibility.
"It's four guys sitting around shooting the bull," Timothy Walch, a historian who edited "At the President's Side: The Vice Presidency in the Twentieth Century," says of McCain, Romney, Crist, and Jindal.
"How candid can they be with one another? Do they have a good sense of humor. Do they communicate well. Do they listen well?"