Political commentators and strategists write with some awe of President Trump’s outrageous, gutsy strategy of ginning up his base with one more attack on black athletes, one more crackdown on Central American mothers and children on the Mexican border, one more assault on Obamacare, one more tariff on imports. They think Mr. Trump’s drumbeat is intensifying loyalty and fervor among Republican partisans and that the Republican Party we used to know is “taking a nap somewhere,” as John Boehner, the former speaker of the House, put it.

Much more worrisome for those of us who think the country needs a blue wave in 2018 is the way Mr. Trump’s strategy appears to be raising his job approval ratings and closing the enthusiasm gap with Democrats that has been a critical element in the handful of off-year elections since 2016. Any wave election worth its salt — like the 1974 Watergate cleansing, the 1994 Gingrich revolution or the 2010 Tea Party shellacking — is produced by the elevated energy and enthusiasm of one party and the demoralization and fracturing of the other.

Well, Mr. Trump’s base strategy is producing precisely that kind of enthusiasm gap in the polls I am conducting for Democracy Corps and its partners, the Women’s Voices Women Vote Action Fund and the American Federation of Teachers. Mr. Trump’s strategy is to continue to build support with the Tea Party supporters and evangelicals who make up a plurality of those who identify as Republicans, but they are by no means the whole of the party. And Mr. Trump shows as much interest in winning over those less enthusiastic Republicans as he does in winning independents and Democrats — which is to say, not much.