Having officially secured the delegates for the Republican nomination, Gov. Romney is the midst of identifying a small group of people, from among whom he will then select his running mate.  Making a short list of qualified potential nominees based on prior experience is, of course, only the first step; a number of political considerations will, and always have, played a role in the final selection.  But whoever is selected, Romney has said, must be "ready on Day 1" to be President of the United States.  Some analysts have concluded this implies a preference for executive experience in potential Vice-Presidents.  Based on presidential history -- and simply what the job of President entails -- Gov. Romney would likely be right to prefer executives, particularly those who have experience as public sector chief executives, as he does himself.  If these individuals also have a background as legislators, as officials in the federal executive branch, and perhaps in the military or law enforcement, they are especially likely to be well prepared to be president.

To find out what kind of experience matters most, I compared the biographies of past presidents with the rankings they have received from historians.  There are several different methods in political science of ranking presidents.  Because it attempts to control for the political bias of academic raters, I chose the 2005 Federalist Society/WSJ rankings and then examined the backgrounds of 39 presidents.  (William Henry Harrison and Garfield are not in the survey because of their short terms; President Obama had not started his term, and I also excluded George W. Bush, because his term of office was incomplete at the time of the survey).  Sixteen presidents are rated as below average or as "failures."  Seven are rated average: Lyndon Johnson, Coolidge, George H. W. Bush, Taft, Madison, Clinton, and Hayes.  (I use this consensus for analytical purposes, but would have, for example, rated Coolidge higher).  Sixteen presidents are rated "above average," "near-great," or "great," and these successful chief executives seem suitable benchmarks for presidential readiness.

Of the sixteen best presidents, ten had been governors: the Roosevelts, Jefferson, Polk, Reagan, Wilson, Cleveland, McKinley, Monroe, and Jackson (as military governor of Florida), and three had been the generals of national armies (Eisenhower, Washington, and Jackson again).  Both governors and top generals can be considered public sector chief executives, acquiring critical skills in staffing and overseeing multiple layers of government subordinates and in working effectively with legislatures, the key tasks of the President.  John Adams, with eight years as Vice-President, and service as the de facto Secretary of War during the Revolution, certainly also had some experience along these lines. Kennedy and Truman, both military officers and U.S. Senators, represent an alternative presidential profile more common in lower-ranked Presidents.  Lincoln -- railroad lawyer, party leader, briefly a militia captain and a one-term Congressman -- is practically unique.  (It was Lincoln's extraordinary personal qualities that arguably put him on Mount Rushmore).

Of the sixteen worst presidents, only three were governors for any appreciable time: Tyler, Carter and Andrew Johnson (Van Buren was governor for 10 weeks while waiting for his appointment as Secretary of State).  Two others, Grant and Taylor, were "national-level" generals who might be classified as chief executives.  Of the rest, Van Buren and Nixon had been Vice-President for several years.  Hoover, like Van Buren, Buchanan and John Quincy Adams, had been in the Cabinet.  Fillmore, Ford, Pierce, Harding, and Benjamin Harrison were all essentially Congressional party leaders with varied government or military service below the chief executive level (though Fillmore and Ford were briefly Vice-President).  Arthur, before his surprise ascension to Vice-President and succession as President, had never held elective office, having been a Civil War quartermaster general, then a party boss and prominent federal civil servant in New York City.

All former presidents, good, bad and in-between, had some governmental "executive" experience, broadly defined.  In fact, almost every President entered office having been (1) a governor, (2) a military officer, and/or (3) an official in the federal executive branch.  The only exceptions, apart from the current incumbent, seem to be the low-rated Warren Harding and Millard Fillmore (whose pre-presidential militia service seems to have been mostly honorary), and both of them briefly held statewide executive posts (as Lt. Governor and Comptroller, respectively).  Lincoln's few months as a militia captain puts him technically within the criteria, and seems to have had psychological importance for him, but its practical value as military experience is doubtful, and so Lincoln should probably be considered an exception as well.

It is the level of executive experience that usually distinguishes good and bad presidents; successful presidents are more likely to have led large public organizations from the top.  By my classification 10/16 of the best were governors, and 3/16 of the worst were; if you include the top generals, the fractions become 12/16 vs. 5/16.  Even with such small groups, the positive effect of being a public sector chief executive is statistically significant (a one-tail test with a p < .02).  This is obviously a very simple analysis, and much more complex correlations can be derived to show the effects of experience on presidential rank.  But it is the chief executive relationship that immediately stands out, and shows why there is a good reason governors and ex-governors supposedly fill up much of Governor Romney's short list (and why some still are hoping for a change of heart from General Petraeus).

Nonetheless, one can be a successful president coming from Congress, where valuable experience is gained by direct participation in a legislative body, as well as through oversight over the federal government.  Three-quarters (12/16) of successful presidents were at some point legislators, and most also were federal office holders, if military service is included (all except Wilson and Cleveland).  Since Governor Romney "has never spent a day in Washington working" (perhaps electorally advantageous this year) and has worked with -- but not in -- a legislature, it would understandable if he picked someone who balanced his gubernatorial experience with someone who has an insider's knowledge of legislative dynamics and of the vast and complicated federal system.  This would follow the precedent of Reagan's selection of George H.W. Bush (who had been a Congressman and CIA Director, among other things) and George W. Bush's choice of Dick Cheney (Congressman and Secretary of Defense, inter alia).  Neither Reagan nor the younger Bush had worked as legislators, and their federal experience was limited to their domestic military service, so their running mates substantively balanced the top of the ticket, and were recognized as doing so.  However, both the elder Bush and Cheney also had significant private and public sector executive experience, which was an intrinsic component of their qualification to be President, and thus, Vice-President.

A military background is likewise common among past presidents, and it has been the subject of some concern that neither of the candidates for Commander-in-Chief this year has one.  Whether it is a true requirement is more open to doubt: half (8/16) of successful presidents did not serve in combat, and include men who led the nation to victory in the Mexican War, the Civil War, and World Wars I and II.  Certainly it would be beneficial for a President to have a military background, because beyond the insights gained by service, it marks a willingness to personally confront public threats, which is a strong indicator of character, courage and patriotism.  But given nearly two generations of a professionalized volunteer force, many highly qualified individuals will be found who have demonstrated these qualities in other ways, for example by work as prosecutors or in law enforcement.  Indeed both backgrounds have value for a president, who is Chief Magistrate just as he is head of the armed forces.

Taken together, a review of the foregoing historical benchmarks suggests a model (civilian) running mate: a current or former public sector executive -- preferably a governor -- who has also worked as a legislator and in the federal government, and who preferably has experience in the military, or perhaps in law enforcement.  The ideal person might be a young Republican governor who started in the state legislature, worked as police commissioner before going into combat as an Army colonel, and also served as an Assistant Secretary in a Republican administration.  However, Theodore Roosevelt is no longer available.  William McKinley's pick of TR as Vice-President worked out well, though, and is worth approximating even if it cannot be easily emulated.

Selecting someone with the right experience is a start, but only a start.  To be a successful president (or to win elections) involves an enormous range of skills that rarely appear on a résumé, principally abilities like communication, charisma and presentation.  So after finding some people with at least of some of the hard-won experience of a Theodore Roosevelt, the next and harder step is to identify a man or woman with at least some of the qualities that allowed Lincoln to transcend his inexperience.  To lead this country through what will be difficult years to come, a president will need Lincoln's capacity to communicate persuasively in many different ways and to many different audiences, and also something of his courage, moral vision, deep insight, unflagging work ethic and profound patriotism.  If Gov. Romney can find a man or woman like Lincoln, then he would be justified in throwing out the rule book on presidential experience; but until somebody like that appears, it make sense to play the percentages, and hire a possible president at least principally on their possession of the background needed for the role of federal chief executive.

Charles N.W. Keckler served in the second Bush administration as a deputy assistant secretary and was a member of the Senior Executive Service.