In a piece timed to coincide with the oral arguments at SCOTUS over same-sex marriage, The Upshot‘s Nate Cohn argued today that a decision to take the issue out of federal and state politics might help the GOP “escape” from a demographic trap on the subject. But it was his analysis of said trap that I found interesting beyond its relevance to this particular debate:

For all of the focus on the “white working class” or the “gender gap” or the urban-rural divide, the real fissure among white voters is along religious lines. The divide between white evangelical Christians and nonreligious white voters is about as large as the gap between white and nonwhite voters, and it dwarfs the education, income, gender or regional gaps.

The Republican advantage among white voters is a product of this division. There are more white evangelical voters than white non-Christian voters, and so the white vote tilts Republican. The remaining white non-evangelical Christian voters, like mainline Protestants and Roman Catholics, have split roughly evenly between the two parties. To the extent that the Republicans have a slight edge among them, it is because of the South.

This was a winning formula for Republicans. There were enough evangelical voters to overwhelm the Democratic alliance between the secular left and nonwhite voters.

That formula is in trouble. The growing nonwhite share of the electorate has played a well-documented role in eroding the viability of the traditional G.O.P. path to the presidency, but the growing number of non-Christian white voters — the religiously unaffiliated, athiests, agnostics, Jews and others — will pose a problem as well.

Among voters under age 45, there were fewer evangelical voters than non-Christian whites, according to a compilation of pre-election surveys of nearly 14,000 respondents. Among 18-to-29-year-olds, white non-Christians outnumbered white evangelicals by a five-point margin. In comparison, evangelicals outnumbered non-Christians by a three-to-one margin among non-Hispanic whites over age 65.

I will note as part of my campaign to fight the habitual marginalization of mainline Protestants and liberal Catholics that even according to Nate’s numbers, non-evangelical Christians outnumber non-Christians in the under-30 demographic. I suppose it’s natural to describe coalitions in terms of the groups that overwhelmingly support it, but you could just as easily say Republicans are in trouble because the white evangelical portion of the population is about to decline precipitously. In any event, there are always going to be enough cultural issues out there to make the Republican dependence on evangelicals a losing proposition in the long run.

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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.