Scratch the surface of almost every projection of a late-developing Romney victory and you will probably find what used to be called “the incumbent rule,” the idea that late-deciding voters generally break against incumbent candidates. What’s ironic is how roles have reversed from the year the “rule” was first really cast into doubt: 2004, when John Kerry did not get the last-minute surge from undecideds that many liberals expected and Republicans scoffed at.

In any event, Mark Blumenthal has done a nice analysis of the “rule,” its demise, and its revival this year by Republicans looking for a thumb on the scales. Blumenthal also usefully offered the first theory I’ve heard for why the “incumbent rule” might have lost some accuracy recently:

Pollsters speculate that the the incumbent rule disappeared because of a change in the nature of campaigns since the ’80s and ’90s. Incumbent candidates have grown increasingly aggressive and are now far more willing to attack challengers early and often, so voters are making their decisions sooner and often on the basis of negative information about challengers.

That was certainly the pattern in this cycle, though something about Romney, or the attacks on him, clearly did make a significant number of voters open to “new information” about him, which is arguably why he got the significant bump after he reinvented himself in the first debate.

But now, when the final undecideds are making up their minds, there’s no particular reason to think the bulk of them are people who’ve already rejected Obama but are still watching those last Romney ads to decide if they’ll bother to vote at all. The “incumbent rule” always made more sense for down-ballot races where exposure to challengers occurred very, very late in the election cycle. The “real” Mitt Romney may still be a mystery, but most voters have made up their minds about him one way or another by now.

UPDATE: The idea that “aggressive incumbents” have neutered the “incumbent rule” was reinforced by Charlie Cook’s latest column on the race for National Journal:

If Obama ekes out an electoral-vote win, look back to last spring and summer, to the Romney campaign’s decision not to define him in a personal and positive way and the Obama campaign’s decision to roll the dice by spending an enormous amount of money to discredit Romney in the swing states, as the factors that led to the outcome.

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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.