10/09/2008 05:12 am ET Updated May 25, 2011

The Vice Presidency: How Close to the Oval Office?

John McCain's surprise choice of Sarah Palin as the Republican nominee for vice president puts into sharp relief two simple questions: just how likely is the vice president to be called upon to occupy the Oval Office? And how likely is it, in particular, that Sarah Palin would become president in the event of a McCain victory?

More than two centuries of presidential history tell us that the ascent of vice president to the White House is a distressingly common event. Of the 43 men who have served as president, 21 percent (9 of 43) died in office of natural (Harrison, Taylor, Harding, Franklin Roosevelt) or unnatural causes (Lincoln, Garfield, McKinley, Kennedy) or were forced to resign (Nixon). Another five vice presidents (Adams, Jefferson, Van Buren, Nixon, George H.W Bush) were elected to the presidency, bringing the overall proportion of vice presidents who ultimately came to occupy the White House to over 30 percent (14 of the 46 men who have served as vice president).

The historical record makes clear that there is a substantial risk that any president might not be able to finish his or her term in office. But John McCain would be no ordinary president; at 72, he would be the oldest first-term president in the nation's history. Though one can only wish Senator McCain a long and healthy life, the national interest requires that we not shy away from an awkward question: how likely is it that a man of his age would die of natural causes during the next four years? According to data from the Social Security Administration, 15 percent of 72-year-old men die before they reach age 76.

Senator McCain, however, is not your average man of 72; he is a cancer survivor, having undergone surgery for skin cancer in August 2000. Physicians who have looked into his medical history have been encouraged by the fact that he has not had a recurrence in the eight years since the surgery; they estimate the chances of a recurrence at this point to be in the single digits. At the same time, they also note that he has at greater than average risk for melanoma and other forms of skin cancer because of his fair skin, extensive exposure to sun at a young age, and his medical history.

Finally, there is the delicate issue, given Senator McCain's age, of the possibility of mental deterioration during the presidency. The risks of both Alzheimer's and stroke increase with age, and declining cognitive function is a not uncommon experience among men and women in their seventies. Minor mental deterioration is often difficult to discern, but should major mental (or physical) decline occur, the 25th amendment provides for the vice-president to assume the duties of the president if the president is "unable to discharge the powers and duties of his office." Had the 25th amendment, passed in 1965 in the wake of President Kennedy's death, been in place earlier in the century, Woodrow Wilson would almost certainly have been replaced by his vice president after suffering a debilitating stroke at age 62 that was hidden from the public.

Taking all the risk factors together, what is the likelihood that a President McCain would have to be replaced by Governor Palin? While no precise estimate is possible, it seems almost certain that the 21 percent figure -- the proportion of presidents over 220 years of American history who have not completed their tern in office -- is on the low side. Given Senator McCain's age, medical history, and the very real possibility of a debilitating episode that would leave him unable to fulfill the duties of the office, the chance that the election of Senator McCain would mean a Palin presidency might be conservatively estimated at more than one in four.

The very thought of a president dying in office or being unable to carry out the duties of the presidency is deeply unsettling and I, like most Americans, fervently hope that that day never comes for the next or any future president. But it has happened before and it will, alas, at some point undoubtedly happen again. As Americans choose their next president, the question of the readiness of the respective vice presidential nominees to meet the awesome responsibilities of the most powerful position in the world may, if the past is any guide to the future, prove all too relevant.