High and growing turnout among white evangelical voters, along with growing Republican margins among white evangelicals, helped Republicans offset this trend. According to the exit polls, white evangelical voters have grown to 36 percent from 29 percent of white voters, while the Republican share of the evangelical vote increased to 79 percent from 69 percent between 2000 and 2012. There is also evidence that Republicans have made modest but smaller gains among white Catholics over the same period.

But as younger, less Christian voters age and their turnout rises, it becomes harder to imagine the Republicans continuing to compensate with higher turnout and support among white evangelicals. In the pre-election polling data, 84 percent of white evangelicals said they would definitely vote, more than any religious or racial group. It’s also hard to see how Republicans will win a much larger share of the white evangelical vote, which supported Mr. Romney by a margin equal to the president’s margin among nonwhite voters.

If Republicans are running out of room to expand their margins among evangelical voters, then additional gains among white voters will have to come from nonevangelicals. And if you assume that cultural issues are the principal reason white voters break so strongly along cultural lines, then further Republican gains among white voters could be well served by a strategic retreat from same-sex marriage.