I think Barro's onto something, but it's not that Trump is a moderate Republican. It's that he's a moderate, full stop. And he's the kind of moderate that really exists, not the kind of moderate Washington likes to pretend exists — which is to say, his policy ideas, such as they exist, are often extreme, but they can't easily be classified as left or right. And there's a market for that.

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In a fascinating paper, Broockman and Doug Ahler look at the results of a survey that gave people seven policy options that ranged from extremely liberal to extremely conservative on 13 different issues.

On marijuana, the single most popular position was full legalization. On immigration, it was "the immediate roundup and deportation of all undocumented immigrants and an outright moratorium on all immigration until the border is proven secure."

These are the kinds of voters Trump could appeal to: voters who hold a basket of opinions that aren't quite represented by either party. Voters who want to deport all unauthorized immigrants while also spending more money on Social Security, or voters who are skeptical of free-trade agreements even as they're virulently anti-abortion. As my colleague Matthew Yglesias wrote, these voters definitely exist:

The Trump worldview isn't just a grab bag of popular issues. It holds together. Political scientists Donald Kinder and Cindy Kam have shown that Medicare and Social Security are more popular among highly "ethnocentric" white voters, while anti-poverty spending is less popular among this demographic. Those "ethnocentric" whites are precisely the ones who are most hostile to immigrants and likely the ones who are most friendly to the anti-Obama "birther" messages that made Trump a political sensation in the first place.