Jonathan Martin’s Politico piece today wherein he asks a bunch of ambitious Democratic governors if they think Hillary Clinton could “clear the field” if she runs for president in 2016 is the first of many along these lines we will likely see in the months and maybe years ahead.

Read the whole thing if you are interested in hearing what the governors are willing to say on the record on this subject (my favorite is the response from Colorado’s John Hickenlooper: “You should be asking Martin O’Malley.”)

But I thought it would be more useful to look at the historical record and ask exactly what it means these days to “clear the field.”

It does not, presumably, mean preempting any competition whatsoever. There will always be someone willing to enter the lists in the early primaries in case the presumed nominee dies or becomes very ill or it is revealed she was in the habit of celebrating Black Masses in her college dorm room. And if the “clearing” candidate is Hillary Clinton (or for that unlikely matter, Joe Biden), the temptation to form an insurgent, anti-establishment campaign will be overpowering to at least one theoretically credible challenger.

The more practical question is whether a candidate as strong as Clinton could obliterate the opposition in the first couple of contests and make the primaries and caucuses a historical footnote.

Back in the days when half or more of the delegates were selected outside the primaries, that was a tough row to hoe. In 1968, Richard Nixon blew away George Romney in New Hampshire, and after a brief stumble in Massachusetts where a dithering Nelson Rockefeller re-entered the race he had left in March, the Tricky One cruised the rest of the way, winning every other primary he entered, usually by huge margins. But it took a late Eisenhower endorsement, plus general election polls showing Nixon doing better in a general election than Rocky, plus various promises to southern conservatives, for Nixon to nail down the nomination in the face of a pincers movement from Rockefeller and Ronald Reagan (who entered the contest on the eve of the convention).

After 1972, when both parties begin requiring that virtually all delegates be selected in party-approved primaries and caucuses, no non-incumbent presidential candidate has really “cleared the field” in the sense of avoiding significant early opposition. Neither of the sitting vice presidents who have run in the primary era–George H.W. Bush in 1988 and Al Gore in 2000–had a cakewalk to the nomination (Bush famously ran third in Iowa, behind Pat Robertson, and Gore was very nearly upset by Bill Bradley in New Hampshire). Conversely, none of the more famous non-candidacies–Ted Kennedy in 1984, 1988 and 1992; Mario Cuomo and Sam Nunn in 1988 and 1992; Newt Gingrich in 1996; Al Gore in 2004; or the many GOP Establishment non-recruits of 2012–happened because of some overwhelming and unbeatable front-runner.

It is entirely possible that HRC is unique in her combination of qualities–universal name recognition, long service to her party, current popularity, solid ideological positioning, and the special advantage of representing the most overdue “historic first” presidency of them all. But even if the rival no one could fault for a challenge to HRC, Joe Biden, fails to take the chance, someone, and probably someone with a resume that commands at least minimal respect, will show up if not at the starting gate, then in the endless preliminaries. And if Clinton puts off a statement of likely candidacy or some other show of force until after the midterms, no telling how many pols with find reasons to spend time in Iowa and New Hamsphire in the interim.

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.