NPR's Audie Cornish talks with Thomas Kidd, a professor of history at Baylor University, about the history of the relationship between evangelicals and political power.

AUDIE CORNISH, HOST:

We've reached the point in the media where the word evangelical has lost a lot of its original meaning. Author Thomas Kidd points this out in his new book "Who Is An Evangelical?"

THOMAS KIDD: I think it is a sign of the politicization of evangelicalism that people who, say, don't go to church would still be willing to say that they're an evangelical. I think that signals that somehow, evangelical now is a fundamentally political term.

CORNISH: Thomas Kidd says prior to the mid-'70s, there wasn't a box to check. But it was shortly after pollsters started actually asking voters about their religious affiliation that we saw the coalescing of a powerful political voting bloc.

KIDD: The transition moment has to be 1976...

(SOUNDBITE OF ARCHIVED RECORDING)

PRESIDENT JIMMY CARTER: My name is Jimmy Carter, and I'm running for president.

(APPLAUSE)

KIDD: ...When one of the major parties nominates an outspoken evangelical, Jimmy Carter, for the Democrats...

(SOUNDBITE OF ARCHIVED RECORDING)

CARTER: All of us - our individual fates are linked.

KIDD: ...As the presidential candidate and obviously eventually became president.

(SOUNDBITE OF ARCHIVED RECORDING)

CARTER: In that knowledge and in that spirit, together, as the Bible says, we can move mountains. Thank you very much.

(APPLAUSE)

KIDD: And one of the most important developments that comes associated with that is that 1976 is the first year that the Gallup organization begins polling about whether people are evangelicals or born again. And it's often not being asked about whether you're an evangelical to see what your spiritual beliefs and practices are but to determine what your political behavior is.

CORNISH: And yet you say it was Ronald Reagan's campaign that unifies white evangelicals and fundamentalists in a way that hadn't been seen since the '20s - so really creating something that feels political.