Has the Republican big tent evolved into a house of worship?

For several years, the two major parties have been moving gradually toward opposite poles: Democrats growing more liberal and secular, Republicans becoming more conservative and religious. But a survey out this week shows just how far and how fast the GOP has gone toward becoming a collection of older, white, evangelical Christians defined as much by religion as by politics.

The nonpartisan Pew Research Center recently released the results of an extensive poll done in 2013 on Americans’ views of evolution. Like other polls, it shows that overall views are stable: Sixty percent believe that humans have evolved over time, the same as said so in 2009.

But within those results, there was a huge shift in the beliefs of Republicans: 48 percent say that humans have existed in our present form from the beginning, compared with 43 percent who say we have evolved, either with or without help from a supreme being. That’s an 11-percentage-point swing from just four years ago, when 54 percent believed in evolution.

Forget climate-change skepticism: Republicans have turned, suddenly and sharply, against Darwin.

How to explain this most unexpected mutation? Given the stability of views on evolution (Gallup polling has found responses essentially the same over the past quarter-century), it’s unlikely that large numbers of Republicans actually changed their beliefs. More likely is that the type of people willing to identify themselves as Republicans increasingly tend to be a narrow group of conservatives who believe in a literal interpretation of the Bible — or partisans who regard evolution as a political question rather than one of science.

The Pew poll also found that the share of Republicans who attend worship services weekly or more is 52 percent, up five points from 2009, and that the proportion who self-identify as conservative is 71 percent, up six percentage points from 2009. The party remains overwhelmingly white, at 86 percent, and the number of those ages 50 to 64 and 65 and older climbed seven points and two points, respectively.

Not all of these changes are statistically significant, but they are consistent with other findings. For example, an analysis of exit polls from the early Republican primaries in 2012 by Ralph Reed’s Faith and Freedom Coalition found that more than 50 percent of participants were evangelical Christians, a record high, up from 44 percent in 2008.

This continues a long-term trend in which both parties are shrinking into smaller entities at opposite extremes. The gap on social issues between Democrats and Republicans (and independents who lean toward one party or the other) has nearly doubled over the past quarter-­century.

Republicans are by far the more ideologically homogenous of the two (seven in 10 are conservative vs. fewer than four in 10 Democrats who are liberal). Because Republicans were already about as religious as they could get, most of the growing gap in recent years has come from Democrats becoming more secular: The share of Democrats who say they never doubt the existence of God has dropped 11 percentage points over the past quarter-century, to 77 percent, while the proportion of Republicans who have no doubt is 92 percent vs. 91 percent 25 years earlier.

That’s what makes the evolution survey extraordinary: The Republican Party is achieving the seemingly impossible feat of becoming even more theological. Democrats and independents haven’t moved much in their views, while Republicans took a sharp turn toward fundamentalism. “The increasing gap isn’t surprising,” says Alan Cooperman, my former Post colleague who is now director of religion research at Pew. “What’s surprising is it’s the Republicans shifting, not the Democrats.”

As a matter of political Darwinism, the Republicans’ mutation is not likely to help the GOP’s survival. As the country overall becomes more racially diverse and more secular, Republicans are resolutely white and increasingly devout. If current trends persist, it will be only a couple of decades before they join the dodo and the saber-toothed tiger.

But give Republicans credit for this: They don’t just doubt the theory of evolution; they’re out to prove it wrong. If they believed in the survival of the fittest, they’d be expanding their racial and ideological diversity. Instead, they’re trying to demonstrate that devotion to God can trump the Darwinian rules of politics.

Keep them in your prayers.

Twitter: @Milbank

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