President Trump speaks to supporters during a rally at the Van Andel Arena on March 28 in Grand Rapids, Mich. (Scott Olson/Getty Images)

A central rationale for President Trump’s unexpected victory in 2016 was his appeal to working-class white voters, particularly in Rust Belt states where he promised to resurrect the sagging manufacturing economy. Trump’s win was often attributed to his ability to speak to the economic insecurity about which those voters were concerned.

Over time, that explanation seemed less compelling as people noted, among other things, that voters who said they were primarily worried about the economy overwhelmingly backed Hillary Clinton. Clinton voters were also more likely to report being economically distressed, including solely among whites.

But still that association with working-class white voters persisted, in part, because Trump’s support from those voters was so robust. If it wasn’t about the economy, what was it about?

New data from Gallup offers a fairly simple explanation: It was a continuation of a steady trend among whites without a college degree toward the Republican Party.

Twenty years ago, working-class whites were about as likely to identify as Democrats as Republicans, while college-educated whites were much more likely to identify as Republicans. The two groups moved mostly in tandem for the next few years, according to Gallup, until 2006. That year, a year when Democrats regained the House, both college-educated and non-college-educated white voters were slightly more Democratic than Republican.

Since then, though, and particularly since shortly after the 2010 midterms — during which the Republicans romped — the groups have diverged significantly.

(Philip Bump/The Washington Post)

In 2016, college-educated white voters were about split between the two parties. Since, they’ve grown much more Democratic, in part thanks to quickly eroding support for Republicans among white college-educated women. White working-class voters have gotten steadily more Republican since 2006.

The Gallup data deals with party identification, but the same pattern shows up in actual voting, too. Exit poll data shows a similar split among whites starting about 10 years ago and widening quickly in recent years. Nonwhite voters, both with and without a college education, continue to vote heavily Democratic.

(Philip Bump/The Washington Post)

There’s another dimension to this that’s worth pointing out. Over time, the density of the electorate that consists of whites without a college degree has dropped, from two-thirds of the electorate in 1980 to about 40 percent in 2018, according to exit polls. The density of the nonwhite vote has grown from about 10 percent in 1980 to 30 percent last year.

(Philip Bump/The Washington Post)

That means, overlaying those numbers onto our party-split graphic, that groups who vote more heavily Democratic are making up more of the electorate in recent years, offsetting the strong Republican support from whites without a college degree.

(Philip Bump/The Washington Post)

Exit polls are election-specific, of course, as the shifts from, say, 2008 to 2010 make clear.

But the point Gallup is making is an important one: The white working-class base Trump enjoyed was already growing increasingly Republican. Maybe he turned more of that base out, but it’s not the case that he was uniquely positioned to benefit from their support. It’s probably not the case, then, that his positions on the economy are what spurred such heavy support from those voters.

They were largely poised to support the Republican 2016 candidate anyway.