The Democratic Party is at risk of repeating the billion-dollar blunder that helped create its devastating losses of 2016. With its obsessive focus on wooing voters who supported Donald Trump, it is neglecting the cornerstone of its coalition and failing to take the steps necessary to win back the House of Representatives and state houses in 2018.

In the 2016 election, the Democratic Party committees that support Senate and House candidates and allied progressive organizations spent more than $1.8 billion. The effectiveness of that staggering amount of money, however, was undermined by a strategic error: prioritizing the pursuit of wavering whites over investing in and inspiring African-American voters, who made up 24 percent of Barack Obama’s winning coalition in 2012.

In spring 2016, when the progressive independent expenditure groups first outlined their plans for $200 million in spending, they did not allocate any money at all for mobilizing black voters (some money was slotted for radio and digital advertising aimed at blacks, but none for hiring human beings to get out the vote).

Predictably, African-American turnout plummeted. According to new census data, 59.6 percent of eligible black voters cast ballots last year, down from the 66 percent who voted in 2012. The problem cannot simply be attributed to the absence of Mr. Obama on the ticket: A slightly higher percentage of black voters, 60 percent, turned out for John Kerry in 2004, than cast ballots last year. In Wisconsin and Pennsylvania, the tens of thousands of African-Americans who voted in 2012 but didn’t vote in 2016 far exceeded the minuscule losing margins for Hillary Clinton.

Nonetheless, Democrats seem to be doubling down on their 2016 strategy. In January, the Senate Democratic Caucus trooped to West Virginia for its annual retreat. According to published reports, the senators heard from panels of voters who had once voted for Mr. Obama but then chose Donald Trump.

The Democratic National Committee’s “Unity Tour,” featuring the committee chairman, Thomas Perez, and Senator Bernie Sanders, included visits to overwhelmingly white states like Kentucky, Maine, Nebraska and Utah. Meanwhile, African-American women — who voted at a rate of 94 percent for Mrs. Clinton last year, the party’s most loyal voting bloc — had to write a letter to Mr. Perez demanding time and attention.

In Georgia’s Sixth Congressional District special election last month, the Democratic nominee, Jon Ossoff, raised a record $23 million and spent dollar after dollar to cast himself as a moderate in a failed attempt to appeal to Republican voters.

The Democratic Party’s fixation on pursuing those who voted for Mr. Trump is a fool’s errand because it’s trying to fix the wrong problem. Although some Democratic voters (in particular, white working-class voters in Rust Belt states) probably did swing to the Republicans, the bigger problem was the large number of what I call “Obama-Johnstein” voters — people who supported Mr. Obama in 2012 but then voted for Gary Johnson, the Libertarian candidate, or Jill Stein, the Green Party candidate, last year (according to the exit polls, 43 percent of them were nonwhite).

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.