Ben Carson, a GOP presidential candidate, speaks to reporters (Tannen Maury/European Pressphoto Agency)

Ben Carson not only wouldn't vote for a Muslim for president; he says he doesn't think the faith is even compatible with the U.S. Constitution. And facing a growing controversy over his comments, the GOP presidential contender's response has been to double down.

In case that's surprising to you, you may want to take a look at the chart below.

The Pew Research Center last year asked people to rate various religions. Perhaps not surprisingly and for reasons that are pretty evident to anybody reading this post, Islam ranked last, slightly behind atheism.

Dig a little deeper, and you'll see that one of the most negative perceptions held by one religion toward another is how white evangelical Christians view Muslims.

Nobody has a worse impression of the Islamic faith and its adherents — not Jewish Americans or even atheists.

White evangelicals are a massive constituency in the Republican presidential primary process, and most notably in Iowa. Nearly six in 10 caucus-goers in 2012 identified as such, according to entrance polls.

Now, having a dimmer view of a certain religion isn't necessarily the same as saying it isn't qualified to be the religion of a U.S. president; but it's not difficult to extrapolate that many potential voters in Iowa could agree with Carson on this one.

There exists a real and valid debate about whether a presidential candidate should advocate for a litmus test when it comes to a president's religion. It's a debate that dates back decades and used to be about Catholicism and, more recently, Mormonism.

But politically speaking, it's not hard to see why Carson sees little urgency to back off his declared position here.