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.

In Delaware, the turnout for Democrats and the unaffiliated matched 2014 levels, while Republican turnout was five percentage points lower. In the end, the partisan composition of the electorate was about the same as in 2016, and Democrats won the race. (For a special election in a state senate race, simply matching previous turnout levels is an impressive feat.)

In Iowa, Democratic turnout was far higher than Republican turnout, improving the Democratic share of the electorate by 14 points since the last midterm election.

The turnout data is harder to interpret in Virginia, where voters do not register with a party. But Republican primary voters outnumbered Democratic primary voters by a somewhat smaller number in both contests than they did in the 2014 elections.

The trend toward higher Democratic turnout appears to be continuing in the April 18 special election for Georgia's Sixth Congressional District, where early voting has recently gotten underway. So far, the party's turnout is running about twice as high as it did at this point in 2014, while Republican turnout is about half what it was.

It would be unfair to judge Republican voters too harshly for their low turnout at this stage — they are trying to decide among 11 candidates. (I wouldn't have voted yet, either.) But the higher Democratic turnout is striking, and if it holds it suggests that the Democratic candidate Jon Ossoff will benefit from stronger party turnout than in the past.

A few elections aren't enough to prove that turnout is really shifting. But there are other signs of higher Democratic enthusiasm, like the millions who marched and protested a day after Mr. Trump's inauguration, or the abundant fund-raising for Mr. Ossoff.[more than $8 million raised for Jon Ossoff to replace Tom Price in Atlanta. Early voting is now!]

Parties out of power have long tended to do very well in midterm elections. It has been less clear why — maybe because of turnout, or because voters swing across parties to check the president.If it's because of turnout, the Democratic midterm turnout problem might just solve itself with a Republican in the White House. If it's not because of turnout, Democrats might be disadvantaged by an unfavorable electorate, even in the sort of election they're supposed to win.

The available evidence is limited, but it suggests that the party out of power enjoys stronger turnout than the party holding the White House. The best evidence comes from Iowa, which has voter turnout data by party registration going back to 1980. It tells a fairly consistent story: Democrats usually have worse turnout in midterm elections, but the Republican edge is greatest when Democrats hold the presidency. The Democratic turnout disadvantage is smaller — or basically nonexistent — when Republicans hold the White House.On average, Republican turnout has been just 6 percent higher than Democratic turnout in midterm elections when Republicans have held the White House, like in 1982, 1986, 1990, 2002 and 2006. Republican turnout has been 17 percent higher than Democratic turnout in midterm elections when Democrats have held the presidency — like in 1994, 1998, 2010 and 2014. The same pattern shows up in the lower-quality data available elsewhere.

It's far too early to say whether Democrats can return to the relative parity they enjoyed in the Bush and Reagan years, especially since the Democratic coalition is younger and more diverse than it was then.

But the history of midterm turnout, the recent special elections, the protests, the donations and the early vote all seem consistent with the same story: The Democrats might be fixing their midterm turnout problem.

This is Nate Cohn's Upshot on April 5, 2017

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April , 2017