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If 2018 Is Like 2017, the House Will Be a Tossup

It’s not obvious that the building Democratic wave will be enough to flip control of the chamber.

Assume, for the moment, that next year’s midterm elections go exactly like this year’s special and general elections. What would happen?

The Democrats would post sweeping gains. They would win the House national popular vote, probably by a wide margin. But the battle for House control would be close. The Democrats might be modest favorites to retake the chamber, but it would probably be fairer to characterize the race as a tossup.

Of course, there’s no reason to assume that next year’s midterm elections will go like this year’s contests. The next year could prove as eventful as the last, and no one can say what the national political environment will be like next November.

But the evidence of Democratic strength has been clear and consistent all year. Democrats in 2017 generally ran far ahead of their recent showings, including ahead of Hillary Clinton’s performance in the 2016 presidential election. President Trump’s approval rating is stuck in the 30s. Democrats have around a 10-point lead on the generic congressional ballot. And the party out of power historically fares very well in midterm elections.

All considered, this year’s election results and the current national political environment are consistent with the possibility of a so-called wave election, like the ones that brought Democrats to power in the House in 2006 and swept Republicans into office in 1994 and 2010.

But Republicans have important structural advantages. They enter the cycle with the advantage of incumbency and a highly favorable congressional map, thanks to partisan gerrymandering and the tendency for Democrats to win with overwhelming margins in heavily Democratic urban areas (and thus “waste” votes).

As a result, it’s not obvious that the building Democratic wave will be enough to flip control of the House.

That’s not because the Democratic showing in 2017 has been any less impressive than in prior wave elections. This year’s results might translate to a Democratic House popular vote win of six to eight points, a margin that would typically be enough to flip control of the House. Indeed, Republicans won the House popular vote by seven percentage points in 1994 and 2010, while Democrats won it by eight percentage points in 2006.

This time, a victory of six to eight points could flip the House — if things broke well for Democrats in the most pivotal two dozen or so seats. But it wouldn’t necessarily do the trick.

The 2017 contests were a mixed bag, ranging from ultralow turnout in special state legislative elections to nationalized congressional races and a high-turnout general election in Virginia. These races often behave very differently, and the way they’re interpreted makes a difference.

The simplest, but deeply problematic, way to look at this year’s races would be to treat them all identically, and look at the change from recent presidential elections.

If you did that, you could argue that the Democrats are poised for a 50-plus seat gain, enough to make them very clear favorites to retake the chamber. That’s in part because Democrats ran ahead of Mrs. Clinton and Barack Obama, but it’s also because they often ran far, far ahead of Mrs. Clinton.

They flipped several extremely conservative seats, as in last Tuesday’s Democratic win in Oklahoma’s 37th Senate District, which supported Mr. Trump by 40 points in 2016. The ability to occasionally run so far ahead of the party could allow Democrats to pull off dozens of wins in districts that don’t even appear to be in contention.

But this is probably not the right way to look at this year’s contests. A better way could drop the expected Democratic gain down to the mid-20s, right near the 24 seats they need to retake the House.

What’s wrong with the simple view?

For one, all of this year’s special elections were open contests, without incumbents on the ballot. Though incumbency has become less powerful over time, it is still worth at least a couple of points to Republicans and perhaps even more.

And, more generally, strange results happen a lot more often in special elections. Low turnout is a big reason for that.

Many ultralow-turnout special elections this year appear to have favored Democrats heavily. Take the first Democratic flip, in New York’s 9th Assembly District. Registered Democrats outnumbered registered Republicans on Election Day, 40 percent to 39 percent, according to data from L2, a nonpartisan vendor of voter files. But in the 2016 general election, Republicans had a 16-point registration advantage, 44 percent to 28 percent. There were even more extreme shifts this year.

This kind of huge difference (17 points) in the partisan composition of the electorate (as measured by registration) is not unprecedented in a low-turnout special election. Only around 10,000 people voted in the race for New York’s 9th; just 4,000 did in Oklahoma’s 37th. It is much more difficult or even impossible to pull off that kind of advantage in a high-turnout election.

That’s probably why the results in the higher-turnout Virginia and New Jersey general elections — in both the governors’ and state legislative contests — looked quite different. The results were far more correlated with recent presidential contests. There were no wild, outlying results, even though there were more individual legislative contests in Virginia and New Jersey than in all of this year’s special elections. This tight relationship between presidential vote choice and state legislative results is reminiscent of typical congressional elections.

In general, Republicans are better off if next year’s congressional election results look less like the special elections and more like the presidential election, since it would tend to lock the Democrats into their geographic disadvantages. The Republicans would then count on the advantages of incumbency to allow enough of the 23 members in Clinton-won districts to survive.

If New Jersey and Virginia represented a better indicator of the midterm playing field, the expected Democratic gain would drop considerably.

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.

Nate Cohn is a domestic correspondent for The Upshot. He covers elections, polling and demographics. Before joining The Times in 2013, he worked as a staff writer for The New Republic. More about Nate Cohn

A version of this article appears in print on  , Section A, Page 14 of the New York edition. Order Reprints | Today’s Paper | Subscribe

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