The possibility that Sarah Palin may well find herself one proverbial heartbeat away from the presidency has flushed out a hitherto buried issue, that of John McCain’s age and health.

George Stephanopoulos raised the subject this Sunday, quoting Obama surrogate Sen. Claire McCaskill’s observation that the next-in-line to “one of the oldest presidents we’ve ever had” has never met a foreign leader.

The second part of McCaskill’s statement strikes me as a blazing-red herring—you can be smart about foreign policy without having met Hu Jintao, and dumb about it while having met, say, Saddam Hussein (which, say, Donald Rumsfeld had certainly done). But McCain’s sudden choice of Sarah Palin certainly does throw into relief not only the question of his judgment—or “management style,” as we call it nowadays in America—but his longevity. McCaskill wasn’t backing down: “I think what we’re talking about is a reality. Other people talk about his melanoma. We’re talking about a reality here that we have to face.”

To which McCain’s surrogate, former Hewlett-Packard CEO Carly Fiorina, said: “I frankly find this disrespectful in the extreme. This is ageism. All you need to do is look at the schedule that John McCain has kept for the last two years to realize that he is one of the most vigorous, most energetic campaigners, frankly, in my judgment, out there.”

It’s fascinating how the Republicans have suddenly discovered all those damnable “-isms”—the ones they used to mock as products of the fevered imaginations of the politically correct—but never mind. This issue deserves some grown-up scrutiny. Enough hiding behind decorum.

No spring rooster myself, I must say I’m impressed by McCain’s energy. Anyone who can maintain the pace of a campaign that runs longer than two baseball seasons and a basketball season combined has my respect in the vigor column. I’m not the first to notice that the campaign—just the campaign—has streaked Barack Obama’s hair with grey. But energy is neither health nor a promise of either longevity or fitness. The citizens are entitled to some straight talk.

Stephanopoulos, to his credit, has raised the issue before, speaking to Lindsay Graham the other Sunday: “So Sen. McCain wins and, God forbid, tragedy strikes. You’d feel confident, safe and secure a year from now if Governor Palin were president?” Everyone, even McCain, knows the second slot on the ticket has four functions: helping number one get elected; presiding over the Senate; attending the funerals of dictators (as John McCain said way back when he had a sense of humor); and finally, overwhelmingly, stepping in if the president dies or becomes disabled.

Let’s go to the actuarial tables. Harold Pollack, faculty chair of the Center for Health Administration Studies at the University of Chicago, has put it this way: “The typical man of Sen. McCain’s age faces a one in seven chance of dying before finishing his [first] term, and a 30% chance of not finishing out a second one.” Mark Kleiman, professor of policy studies at the UCLA School of Public Policy and Social Research, notes that “the incidence of severe disability over the next four years of McCain’s life is…7 percent.” In sum, then, to quote Kleiman: “the probability that Palin would have to take over at some point in McCain’s first term is 21%.”

These are numbers for typical American men. You may, if you like, remark on the X chromosome he inherited from his mother, who is evidently healthy on the far side of ninety-six; but then you are honor-bound to note that he has already outlived his father, who died at seventy. And surely the McCain’s longevity is not improved by his history of four malignant melanomas.

Long live Sen. McCain! But think about that 21 percent. You get better odds with Russian roulette.

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Todd Gitlin , who teaches journalism at Columbia, is the author of a new book, Occupy Nation: The Roots, the Spirit, and the Promise of Occupy Wall Street.