Voters cast their ballots in the Eastmont Community Center in Los Angeles on Nov. 6. (Kyle Grillot/Reuters)

Economic growth is at a three-year high, and unemployment is at a 49-year low, so President Trump, of course, is focusing his midterm message on the supposedly apocalyptic dangers emanating from a caravan of migrants 700 miles away.

That’s not to say that Trump never touts his economic record. He does. It’s just that it’s not as often as we might have expected, considering how well things are going right now.

Although maybe it shouldn’t be too surprising. The economy, after all, was adding a very healthy 171,000 jobs a month in 2006 before Republicans lost 31 House and six Senate seats. And it added an even more robust 245,000 jobs a month in 2014 before Democrats dropped 13 House and nine Senate seats.

When it comes to the midterms, then, it sure seems like it’s not the economy, stupid. But, as we’ll get to in a minute, it also is, partly, the economy. Midterm elections tend to first and foremost be a referendum on the president, with the economy certainly playing a part but not in the way, or maybe as much, as one might think.

The simple story is that voters don’t really care about GDP growth or the unemployment rate in and of themselves, but rather what those numbers mean for themselves. While there’s no real relationship between how high unemployment is and how the president’s party does in the midterms, there is some relationship between how people’s pocketbooks are doing and the election results.

University of Denver political scientist Seth Masket has found that, specifically, what seems to matter is how much people’s incomes have grown (after taxes and inflation) from the third quarter of the year before the election to the second quarter of the year of the election. It explains about 20 percent of the midterm results, according to Masket. If you add in the president’s approval rating, you can explain about 57 percent of those results

Maybe the best way to think about this is that the economy helps determine how favorable the political ground is for the president in the midterms, but the election battle itself is usually over what the president has done.

That leaves Trump with two problems. The first is that for all the good economic news, wage growth has still been pretty mediocre. So the economy might not be quite the winning issue that it seemed.

The second is that while Trump ran as a populist — he promised to put a lot of money into infrastructure, perhaps raise taxes on the rich and provide health insurance “for everybody” — he’s governed as a plutocrat instead. Indeed, he’s done or tried to do the opposite of everything he promised.

It’s no wonder Trump wants to talk about something else.