In Wisconsin, for example, the Democratic vote total dropped by nearly 235,000, while Mr. Trump got only about the same number of votes as Mr. Romney in 2012. The bigger surge in that state was for Mr. Johnson and Ms. Stein, who together won about 110,000 additional votes than the candidates of their respective parties had received in 2012. And in Michigan, which Mrs. Clinton lost by fewer than 11,000 votes, the Johnson-Stein parties’ total increased by about 202,000 votes over 2012.

The Democratic Party committees and its allies are likely to spend more than $750 million on the 2018 midterms. Will they spend it fruitlessly trying to lure Trump voters, or will they give uninspired black Democrats a reason to vote and offer disaffected Obama-Johnstein voters a reason to return to the fold?

Democrats have an opportunity in 2018 because of the significant enthusiasm gap between the parties. By concentrating their firepower on inspiring, organizing and mobilizing people who voted for Hillary Clinton to vote again in 2018, Democrats can take back the House and also win the governor’s office in six key states — Arizona, Florida, Georgia, Ohio, Michigan and Wisconsin — for a fraction of their $750 million budget, less than $100 million.

In the congressional special elections and primaries for governor this year, just 39 percent of the Republicans who voted in the 2016 presidential election came back out to vote this year, while 57 percent of Democratic voters returned to the polls. That’s a normal pattern for midterm elections: The in-power party almost always sees a sizable drop-off in enthusiasm.

Too many Democrats sit out midterm elections (in 2014, drop-off was slightly over 40 percent). Those infrequent but Democratic voters hold the key to the balance of power in America. Democrats need to pick up 24 seats to take control of the House, and there are 28 Republican-held seats in districts Hillary Clinton won or nearly won. If Republican turnout drops by the 36 percent that it did the last time a Republican held the White House, Democrats need to get 951,000 drop-offs to vote again in those 28 districts. Civic engagement experts have found that an effective canvassing and mobilization program costs about $50 per infrequent voter who actually casts a ballot.

By that metric, it would cost $47.6 million to get enough infrequent voters to the polls in the 28 congressional districts that will determine which party holds the House. In the six battleground-state contests for governors, the cost to bring out the necessary number of infrequent voters is $42.1 million.

The country is under conservative assault because Democrats mistakenly sought support from conservative white working-class voters susceptible to racially charged appeals. Replicating that strategy would be another catastrophic blunder.