As noted earlier today, one important variable for Democrats on November 6 is the size of the under-30 vote, a huge asset in 2008. But as a moment’s reflection should indicate, it’s a bit more complicated than that: a new cohort of voters just now qualified to participate has entered the system, which means that if Dems can hang on to a disproportionate share of the voters who were just under 30 in 2008, a falloff in “youth vote” turnout could disguise a growing partisan advantage. Nate Cohn runs the numbers:

[I]f turnout among these voters is down 18 percent—and that’s beneath 2004, by the way—the total number of young, disproportionately non-white, and Obama-friendly voters actually increases from 23.5 to 25.7 million….

Even in this relatively low-turnout scenario, 6.5 million new 18-22 year olds will enter the electorate and they can go a long way toward helping Obama compensate for declining turnout among ’08 voters or an increase in conservative turnout. If they vote 63-37 for Obama, the president would net 1.7 million voters.

This is why “generational” elections like 2008 matter a lot. A rising generation tends to cling to its initial partisan leanings, and is buttressed by new voters with similar tendencies. Old voters don’t “check out” as quickly as new voters “check in,” but the net effect is gradually in favor of whoever captures the imagination and allegiances of the younger cohorts.

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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.