Donald J. Trump’s low approval ratings and the palpable enthusiasm of progressives nationwide have Democrats dreaming of a big win in next year’s midterm elections. But to pull that off, they’ll need to overcome one of their biggest challenges of the last decade: low turnout in off-year contests.

The Democratic turnout in those elections has been extremely weak — worse than many public analysts have suggested. Democrats have depended on young and nonwhite voters, two groups that produce low turnout in midterm contests. Nationwide, Republicans were more than 20 percent likelier to vote than Democrats (defined by party vote history and registration) in 2010 and 2014, according to an Upshot analysis of voter file data from the company L2.

But there are early signs this could be changing. If it does in 2018, it will be consistent with a longer-term trend in which the party out of power benefits in midterm elections, seemingly from a stronger turnout.

Democrats have fared well in recent special elections, and they have turned out in strong numbers in the four contests where complete turnout numbers are now available: a relatively uncompetitive special election in Iowa’s 45th State Senate district in December, two January contests in Virginia, and Delaware’s 10th State Senate district race in February.