Here’s the answer, from a fascinating interview with Dr. Robert Jones, the CEO of the Public Religion Research Institute, who has done a lot of detailed, fine-grained polling:

In a nutshell, I think these voters are best understood not as values voters, not even as Tea Party voters, but as nostalgia voters, these voters that are looking back to — they’re culturally and economically disaffected voters that are anxious to hold on to a white conservative Christian culture that’s passing from the scene. I think that’s the core of who his supporters are. I think it’s highly doubtful that there are enough of those voters out there to get him across the finish line no matter who the Democratic nominee is in the general election. But there may be enough of them, and it looks like there are, to get him across the finish line to be the Republican nominee.

Lots of detail in the interview. Worth reading in full.

An alternative view is that they are people who are in revolt against the ‘Establishment’. But what is that, exactly? Here’s Nassim Nicholas Taleb’s take on it: