For the midterms, I devised a new forecasting model informed partly by this new paradigm of voter behavior. It was as accurate as the best in the forecasting business, and my predictions were made months ahead of the others. That’s important, because it is already telling us what we can expect from the 2020 election.

For the 2018 midterms, my model predicted a large partisan surge for Democrats. I identified America’s suburbs as ground zero for a political realignment away from Republican House candidates. The realignment was fueled by two things: One was conventional — the movement of disaffected independent, or swing, voters away from the president’s party, which has happened in every midterm election since 2006.

The other can be tracked to the mobilization of negative partisanship in driving turnout from Democrats who usually sit out midterm elections. By identifying Republican-held districts with both a reasonably competitive partisan electorate and a large number of college-educated voters who could form a Democratic turnout swell, I predicted — in July 2018 — that negative partisanship would allow Democrats to pick up 42 House seats and sweep “Reagan country” in Orange County, Calif. At first, my model was an outlier, but by Election Day, the FiveThirtyEight “classic” forecast, Sabato’s Crystal Ball and the Cook Political Report all agreed with my forecast.

Motivated by the threat posed by the Trump administration, casual Democratic voters, especially college-educated women, have been activated since Mr. Trump’s election and will remain activated so long as the threat he presents to them remains. And the complacent Democratic electorate of the 2010 and 2014 congressional midterms as well as the 2016 presidential election is gone (for now). It has been replaced by a galvanized Democratic electorate that will produce the same structural advantage for Democrats that manifested in the 2018 midterms.

The surge won’t be uniform. Democrats will win big in more urban, more diverse, better-educated and more liberal-friendly states and will continue to lose ground in other states like Missouri. Although Mr. Trump may well win Ohio and perhaps even Florida again, it is not likely he will carry Michigan, Wisconsin and Pennsylvania in 2020. Look at the midterm performance of statewide Democrats in those states. And his troubles with swing voters, whom he won in 2016, will put Arizona, North Carolina and perhaps even Georgia in play for Democrats and effectively remove Virginia, Colorado, Nevada and New Hampshire from the list of swing states.