Midterm elections are all about turning out base constituencies. Over the last few decades, there have been few more reliable voters for Republicans than white evangelical Protestants. This year, however, GOP candidates may be getting less help from this group—not because white evangelical Protestants are becoming less supportive or less motivated, but simply because they are declining as a proportion of the population, even in Southern states.

White evangelical Protestants have remained a steadfast Republican constituency in both presidential and midterm congressional elections ever since the Reagan presidency, which marked what political scientists Merle and Earl Black dubbed “the great white switch.” In 2008 and 2012, roughly three-quarters of white born-again Christians supported GOP nominees John McCain (73 percent) and Mitt Romney (78 percent). In the 2010 midterm election, similar numbers of white born-again Christians (77 percent) supported the GOP House candidate in their districts.

During the heady days of evangelical prominence in the 1980s and 1990s, white evangelical Protestant leaders frequently noted the decline of their more liberal mainline Protestant cousins, but now white evangelicals are seeing their own populations shrink. In recent years, for example, the Southern Baptist Convention, the largest evangelical denomination in the country, has reported steady declines in membership and new baptisms. Since 2007, the number of white evangelical Protestants nationwide has slipped from 22 percent in 2007 to 18 percent today.