Democratic congressional challengers have posted very impressive fund-raising tallies so far this year. In the last quarter, nearly three dozen Republican incumbents were outraised by at least one Democratic challenger — an astonishing number against sitting members of Congress. The breadth of Democratic fund-raising is also strong.

But there’s an important pattern under the big numbers that will shape the battleground in the 2018 midterm elections. So far, nearly all of the biggest Democratic recruiting struggles have been in working-class areas. And Democrats might have too many challengers successfully fund-raising in the most affluent districts.

Democrats have debated extensively about whether they ought to focus on winning back working-class Trump voters or on expanding their gains in diverse, well-educated Sun Belt suburbs. This can be a false choice: They can do both to some extent, especially in congressional elections where individual candidates can run campaigns well suited to their districts. But Democrats, who need a net gain of 24 seats to retake the House, won’t have the option to target districts they held as recently as a decade ago if they can’t find viable challengers.

There is not yet a strong Democratic challenger in David Valadao’s district (the 21st) in California’s Central Valley, the nation’s least-educated Republican-held congressional district (going by the percentage of those with a college degree). It broke for Hillary Clinton by 16 percentage points last November. There isn’t a strong challenger in John Katko’s upstate New York district (the 24th), where Barack Obama won easily in 2012 and where Mrs. Clinton won in 2016. These races remain rated as “likely Republican” by the Cook Political Report.