One of the hoary assumptions about presidential elections that used to be intoned regularly as an ‘iron law” was the so-called “incumbent rule:” the idea that in contests involving incumbents, undecided voters tend to break sharply in favor of challengers late in the cycle. The “incumbent rule” has been an important prop for the still powerful “referendum rule” holding that such elections are always and invariably up-or-down votes on the record of the incumbent, with the identity, ideology or agenda of the challenger being essentially irrelevant.

Together, the “incumbency” and “referendum” rules paint a picture of an electoral dynamic in which voters make up their minds about the incumbent at some point well before Election Day and then stampede into the challenger’s column. For obvious reasons, this is the picture tht pro-Romney spinners–most notoriously Dick Morris–are busy projecting in every utterance.

I mention all that by way of suggesting you read Nate Silver’s latest FiveThirtyEight post, which (although Nate is as always cautious) pretty much buries the “incumbent rule.” He does find that early polls often understate challengers’ final performance because (a) they are engaged in primary battles that artificially depress their support levels among members of their own party, and (b) they are not as well known early on as incumbents. We’ve seen that phenomenon in 2012, when Mitt Romney was running well behind Obama in general election trial heats back in the early spring before he put away Rick Santorum and began to consolidate Republican support. But Nate suggests that incumbents are as likely to get a late “bounce” as challengers, based on looking at every presidential election since 1968.

It’s important to understand that the “incumbent rule” is the kind of hypothesis that is relatively simple to test empirically. The “referendum rule” isn’t, for the obvious reason that it’s much harder to deduce why voters wound up supporting a challenger as opposed to an incumbent. A swing voter who says he or she decided to “fire” an incumbent for poor job performance in order to “give the challenger a chance” may have actually compared the two candidates and found the challenger’s message–which of course will always involve a recommendation that the incumbent be “fired” for poor performance–more compelling. And lest we forget, the vast majority of voters, particularly at this heavily polarized juncture, are casting ballots on the basis of partisan affiliation and aren’t making any of the calculations that analysts so often attribute to them.

Ed Kilgore

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.