If you simulated the results of the 2018 election, using 2017 elections, but considered incumbency and weight contests by total vote — which places considerable emphasis on Virginia and New Jersey — the Democrats would be poised to win about 27 Republican-held seats. That would put the Democrats just over the 24 seats they need to retake the House, provided they held all or nearly all of their seats, as they are expected to do. But the margin of error is plus-or-minus eight seats, so you could not rule out Republican control. And remember, this is the margin of error only if you make the big assumptions that the results go as they did in 2017, and that the New Jersey and Virginia races should be weighted in proportion to their far larger turnout.

Neither of those assumptions is ironclad. The case is good for emphasizing the higher-turnout general election results with incumbents, but it effectively limits the most relevant data to just two electorates. They were state, not federal elections.

Over all, Democrats did about three points worse in the general elections than in the special elections, even after adjusting for the lack of incumbent opponents in Virginia and New Jersey. If one could dismiss that three-point difference as an artifact of low turnout, it would be easy enough to focus on Virginia and New Jersey. But it’s not so simple. Some of the strongest Democratic showings of the year came in the relatively high-turnout special congressional elections, like in Montana or in Georgia’s Sixth. One could credibly argue that those special congressional elections were the contests most like next year’s midterm elections.

If you emphasized the special congressional election results, and believed that Democrats would only do about two points worse in races with incumbents (the difference in Virginia and New Jersey), the Democrats might be poised to pick up more than 40 seats. My view is that there haven’t been nearly enough of these contests to be confident that Democrats are on track for such significant gains, or to be sure that the incumbency bonus for Republicans is so small. But it is at least an argument for a larger Democratic gain than 27 seats if next year’s elections resemble this year’s contests.

Next year’s contests will occur in a very different context, even if the national political environment is fundamentally similar. The races will be federal elections, usually with incumbents. The midterm fight will probably receive considerable national and local press coverage in the weeks — if not months — ahead of Election Day. Mr. Trump will loom over it all, and at least one candidate in nearly every contest will have an incentive to nationalize the race.

What’s clear, though, is that this year’s election results are fully consistent with a wave election. This shouldn’t be a surprise. It’s what’s supposed to happen in an off-year election when the president’s approval rating is in the 30s. If that’s true a year from now, the Republican House will be in jeopardy.