The 2017 Elections Suggest Incumbency Won’t Save Republicans In 2018 Filed under 2018 Election

Tuesday was a great night for Democrats. Their candidate for governor of Virginia, Ralph Northam, ran away with a 54 percent to 45 percent win. The party gained complete control of two state governments, New Jersey and Washington. But perhaps most significantly, the party picked up at least 15 seats in the Virginia House of Delegates, with recounts likely in four districts that will determine control of the chamber.

True, it may seem like possibly, maybe flipping one state legislative chamber doesn’t stack up to those other accomplishments. But more than their implications for governance of Virginia, Tuesday night’s legislative results should cheer Democrats because of what they represent: the best bellwether to date of the 2018 congressional elections.

Ahead of Tuesday, liberals were already rightly bullish about their midterm prospects because of the clear pattern of Democratic overachievement in special elections for congressional and state legislative seats in 2017. Historically, special-election results, in the aggregate, have been a pretty good omen for how a party will do in the following midterms, but it’s best not to rely too heavily on such a small sample of fickle data points (these elections are, by definition, special). And now with Tuesday’s 100 elections for the Virginia House of Delegates and 40 elections for the New Jersey state Senate, we have more than twice as much legislative-election data as we did coming into the week.

As FiveThirtyEight contributor David Wasserman has pointed out, these regularly scheduled elections are better approximations of 2018’s U.S. House elections. Unlike special elections, in which unpredictable turnout levels can favor one party and skew the results, Virginia’s and New Jersey’s elections occurred concurrently with those states’ gubernatorial elections (36 states will select a governor in 2018), a pairing that typically leads to more predictable turnout. Perhaps most importantly, these legislative elections were predominantly not open-seat races, as vacancy-triggered special elections obviously are. Tuesday was our first chance to see whether Democrats could still beat expectations when going up against the powerful force of incumbency advantage.

The answer — as we’d expect with actual incumbents on the ballot — is that Democrats did well on Tuesday but didn’t outperform expectations to the degree they have in special elections this year. On average, the party’s candidates for Virginia House of Delegates and New Jersey state Senate won by margins that were 3 percentage points better than their district’s partisan lean.

How Democrats did in the Virginia House of Delegates Democratic performance in districts relative to each district’s partisan lean View more! Only races that pitted one Democrat against one Republican are included. 2017 election results are unofficial and as of 2 a.m. Eastern on Nov. 8.

A district’s “partisan lean” is the average difference between how the district voted and how the country voted overall in the last two presidential elections, with 2016 weighted 75 percent and 2012 weighted 25 percent. Sources: Associated Press, Daily Kos Elections

How Democrats did in the New Jersey state Senate Democratic performance in districts relative to each district’s partisan lean View more! Only races that pitted one Democrat against one Republican are included. 2017 data are unofficial results as of 2 a.m. Eastern on Nov. 8. Sources: Associated Press, Daily Kos Elections

Unsurprisingly, incumbency was a major factor in how Democrats fared. In open seats and districts with Democratic incumbents, the party outperformed the partisan lean by a healthy amount — although not by the same 14 points by which they beat the partisan lean in special state legislative elections the last time we examined this question. But in districts with Republican incumbents, Democrats basically matched the district’s partisan lean.

A blue night Average Democratic performance relative to each district’s partisan lean, by incumbency status View more! Only races that pitted one Democrat against one Republican are included. Results are unofficial and as of 2 a.m. Eastern on Nov. 8. Sources: Associated Press, Daily Kos Elections

In Virginia specifically, the main reason that Democrats were able to win so many House of Delegates races on Tuesday was that so many Republicans were occupying blue seats. Going by our partisan lean metric, 50 House of Delegates seats are naturally Republican-leaning and 50 are naturally Democratic-leaning; going into Tuesday, Republicans held all 50 of the former and 16 of the latter. If the current leader wins in every race, though, Republicans would hold only two Democratic-leaning seats in the next House of Delegates (Democrats would also hold two Republican-leaning ones). Put another way, Tuesday was a corrective election.

Basically, the national environment (as expressed by the generic ballot and other indicators) was friendly enough to Democrats that it neutralized Republicans’ incumbency advantage on Tuesday. If this holds in 2018, that may be enough to put the U.S. House in play. And, of course, not all seats Republicans will be defending will feature incumbents, further brightening the outlook for Democrats. The more Republicans continue to retire from Congress, the clearer the Democrats’ path to a House majority will become.