Political insiders and pundits continue to talk up Kathleen Sebelius and James Webb as ideal running mates for Barack Obama. And I understand why. Unlike most of the other names that get tossed around--Joe Biden, Ed Rendell, and, of course, Hillary Clinton--both Sebelius and Webb are fresh faces. Quite apart from their apparent abilities to win over more conservative voters, they would--by their mere presence on the ticket--underscore Obama's message of change. The slogan practically writes itself: New Leaders for a New Era, or something like that.

But while excitement is good, it also comes with risk. As I've written before, I think the most important priority in picking a running mate should be choosing somebody clearly qualified to be president. A close second to that is the ability to perform the job of vice president, a job description that includes serving as a capable surrogate and helping the president to govern more effectively.

Do these two candidates live up to that standard? Sebelius' record in state government is impressive, yes, but it's entirely in state government. She's obviously intelligent and has shown plenty of good judgment. Still, how can anybody--including Sebelius--know what her foreign policy views will be when she's never had to formulate them except as an intellectual exercise?

Webb has the opposite problem. He'd lend an Obama administration more credibility and expertise on national security. But who knows whether he is up for campaigning, and serving as an effective elected official, when his experience in those roles goes back all of two years? (Here's Slate's Timothy Noah with some pretty good reasons to worry.) And while his conversion from the GOP makes for a great storyline, it also raises some questions about partisan loyalty--questions that may reflect perfectly well upon Webb personally, but don't necessarily make him the most reliable of Democrats in the long term.

I realize there's an obvious rejoinder to this: You could have said some of the same things about Barack Obama when he first declared his candidacy. He was a newcomer to the national stage. He had virtually zero foreign policy experience. And so on.

But that was sixteen months ago--and, since then, we've learned a lot more about him. All of those debates may have been tedious and, on occasion, downright depressing. But by the time they were over, we'd gotten a pretty thorough picture of Obama's specific issue positions as well as his broader governing philosophy, spanning virtually every major issue likely to confront the next president. The grueling nature of the campaign also tested his leadership and management abilities, even under intense political pressure. We don't know everything we need to know about Obama, I realize, but we know an awful lot--enough, I think, to render an intelligent judgment about his fitness for the presidency.

The public won't get sixteen months to vet Obama's vice presidential candidate. Neither, for that matter, will the media, the party establishment, or even Obama himself. And yet the stakes in this choice are incredibly high. If my math (and history) is right, fourteen of the fifty-six forty-six men who've served as vice president--exactly one in four just under one-third--went on to become president, either by assuming office or running subsequently on his own. Even those who didn't become president someday frequently became dominant political figures in their era.

This isn't to say a relative newcomer like Sebelius of Webb should be out of the running. Far from it. Obama and his selection team will presumably get to know the possible candidates, and their histories, better than outsiders like me ever could. And based on that information, Obama might be able to determine that either Sebelius, Webb, or even some other novel figure is up for the job. But insofar as people outside the process--like those of us in the media--are hyping these two candidates, we should remember that sometimes boring and predictable can be virtues, too.

Edit: Corrected my hilariously poor arithmetic. 

--Jonathan Cohn