The economy invariably ranks among the top issues on the minds of voters in presidential elections. At the moment, it appears to offer President Trump a meaningful tailwind.

But how big is that tailwind? Fortunately, economists have worked hard to develop models for predicting election outcomes, and according to one of the best of these, it should be quite large.

One of the first — and perhaps still the best — of these models was created by Ray Fair, a professor at Yale. He found that the growth rates of gross domestic product and inflation have been the two most important economic predictors — but he also found that incumbency was also an important determinant of presidential election outcomes.