After more than two decades on the quadrennial short-list, the idea of former Georgia senator Sam Nunn as vice president has become a cliché. And knocking him down is easy. You know the rap. He's too old (69), too rusty politically (out of the Senate since 1996) and too conservative (he helped design the don't-ask-don't-tell policy on gays in the military in the early 1990s).  Plus, he's dull.

But Nunn may be the best pick for Barack Obama in a year when the presumptive Democratic nominee has no obvious choices. If some potential candidate could immediately offer strength to Obama on the economy, he should pick him or her. But no one fits that bill, which leaves the field open to a foreign policy heavyweight who could help compensate for the slim resume of a freshman senator. The notion that Obama would be better off pretending this weakness didn't exist—that he should double down on change with a young running mate—ignores the readiness bar Obama still needs to clear.

"The odds are very much against it and I don't expect to be offered it," Nunn said July 3 at the Aspen Ideas Festival, looking a lot younger and more fit than, say, John McCain.   That's a pitch-perfect version of the coy dodge expected of all serious candidates for the job.

The main reason Nunn has a chance is that Obama has told his advisers that he won't choose anyone who lacks the stature to be perceived immediately as a plausible president. This makes any short list much shorter. Kathleen Sebelius and Ted Strickland, for instance, are good governors but they just aren't going to make that cut; Nunn's foreign policy experience, unquestioned intelligence, and big thinking assure that he does.

In Nunn's case, out of the Senate doesn't mean out of the action. His record in the 12 years since he left is impressive. Nunn and Sen. Richard Lugar have, with little public attention, managed to reduce the greatest security threat in the world—loose nuclear weapons in the former Soviet Union.  The Nunn-Lugar initiative has been a huge success and a Nobel Prize is a distinct possibility.

When Obama came to the Senate, he locked onto Nunn-Lugar as his primary foreign policy interest. The only major piece of legislation that bear Obama's name is the Lugar-Obama Initiative, which secures loose conventional weapons using the same protocols. Don't underestimate the importance to Obama of selecting a vice-president who shares his world view.

Last month, Nunn, who chairs the Nuclear Threat Institute, joined with several formers secretaries of state and defense in writing a Wall Street Journal op-ed piece that offers a vision of a world free of nuclear weapons altogether—and specific proposals for how to get there. Nunn goes further than some of his co-authors and advocates the once-heretical idea of the United States allowing cameras and international inspectors inside its nuclear weapons facilities.