The silent Republicans may also fear that it will seem like they’re taking sides against religion if they embrace evolution. Both McCain and Romney were able to avoid that danger, though, in essence by saying that they believed that evolution was part of God’s plan. (Another 31 percent of the public takes that view, again per the Gallup Poll.)

In refusing to address evolution, Walker said, “That’s a question a politician shouldn’t be involved in one way or the other.” It’s true that vanishingly few questions of federal policy turn on whether human beings have evolved from other species. But it does appear that for at least some Americans, belief in that proposition is a marker of a candidate’s acceptance of science and modernity, and rejection of it marks a candidate as anti-intellectual or just plain dumb.

Which raises another possible explanation for Republicans’ unwillingness to answer questions about the issue. They may themselves belong to that large anti-evolution bloc and not wish to turn away voters for whom a candidate’s belief in evolution is important. But for many of these voters, silence on evolution is nearly as bad as a candid rejection of it.