The odds are good that the next president will be a senator, which raises an obvious but important question: why are Warren Harding and John F. Kennedy the only two presidents who won election as sitting senators?

The most complete treatment I’ve seen of the subject is Barry Burden’s 2002 Political Science Quarterly article (PDF), which was picked up by David Broder in the Washington Post. Here is Burden’s conclusion:

Contrary to the conventional wisdom found in newspaper stories and academic journals, senators have been poorer presidential candidates than one might expect. Many of them have run for president, but surprisingly few have earned their parties’ nominations and even fewer have won the general election. Former senators do well, though much of their success is attributed to being selected as vice-presidential running mates. The group of presidential candidates most analogous to senators — governors -— make up the second largest share of the contender pool and perform better than senators when they run. Though senators are probably at least as likely as governors to harbor progressive ambition for the presidency, I argue that their differing success rates are due in part to deeper campaign investment by the typical governor. Governors also benefit from more careful self-selection, with those from larger states, especially in the South, more likely to run. It seems evident that senators, especially sitting senators, have not done as well at the nomination and general election stages as they should have, given what we know. The Senate has seldom been the presidential incubator or nursery it ought to be given the ambition, visibility, resources, and records of both current and former members of the institution.

Of the four general explanations offered for the gap between expectations and performance, some have received more support than others. The “authority and the issues” and “expectations” arguments seem the least potent. There are at least two reasons to be skeptical of these. First, all members of Congress do not perform equally well. Representatives and senators do have similar success rates in the long run, but members of the House differ significantly from senators on several predictors of success. Cabinet officials, business people, activists, and mayors have done about as well too, so there does not appear to be a liability unique to legislators. Representatives should be less successful than senators, since their constituencies are smaller. They serve in a chamber of 435 rather than 100 and have generally spent less time climbing the hierarchy of offices to get where they are. Second, current senators are less successful than former senators. If there is a congressional aura that hurts legislators when they run for president, it does not extend to members who have left Capitol Hill.

I am more optimistic about the “office structure” and “candidate pool” arguments. They have clear connections to the investment and ambition theory and garner tentative support in the rudimentary analysis done here. They explain why the pools of senators and governors are not quite the sizes that we assumed if former office-holders are included. Both arguments also rely on notions of supply and demand and strategic decision making by elites, two crucial elements of any study of presidential selection. Mysteries about ascension of particular candidates to the White House remain of course, but revealing new facts and proposing explanations for them that are tied to theory have proved useful.

Other theories? Should we expect 2008 to be different? Right now, the Tradesports market puts a 56% probability on McCain, Obama, or Clinton winning the general election.