One economic model predicting presidential elections using just GDP growth shows that the economy must grow at an annual rate of 2.5% in the second quarter of the election year in order for the incumbent party to win. Based on the latest release showing Q2 growth at an annual rate of 1.2%, Trump has virtually won.
The model created by Ray Fair at Yale University agrees, according to a story on NPR: "It's based on economic growth per capita in the four years before the election. According to Fair, because of the sluggish growth in this recovery, his model now predicts the Republican candidate will win. Fair's model has picked the winner in all but two elections since 1916."