The 5 Big Takeaways From Our House Forecast

By Nate Silver, FiveThirtyEight

18 August 18

emocrats are favored to gain control of the House of Representatives in this year’s midterm elections, according to the FiveThirtyEight forecast model. But — a very FiveThirtyEight-ish sentence follows — the range of possible outcomes is wide and Democrats’ prospects are far from certain. Relatively small shifts could allow Republicans to keep control of the House, or could turn a blue wave into a tsunami.

What’s behind all of this? Our methodology post goes into a lot more detail about how our forecasts are calculated. But that explanation is rather abstract, so in this article, I’m going to focus on how these factors are playing out given what we know about the political environment this year.

Theme No. 1: A broad consensus of indicators point toward Democrats performing well

In contrast to our presidential forecasts, which are heavily dependent on polling, our House model uses a broad mix of polling and non-polling indicators, including factors such as fundraising totals and historical trends in midterms. Those indicators look both pretty good for Democrats and remarkably consistent with one another:

• The Lite version of our forecast, which focuses as much as possible on district-level and generic ballot polls, projects Democrats to win the popular vote for the House by 7 or 8 percentage points.
• The Classic version of the model, which incorporates a lot of non-polling metrics such as fundraising and past voting in each district, also shows Democrats winning the popular vote by 7 or 8 points.
• The generic ballot, which influences all three versions of our forecast, has generally shown Democrats with a lead of … 7 to 8 percentage points.
• And finally, our model calculates a starting assumption about the race based on historical trends in midterms since 1946 and presidential approval ratings. It also implies that Democrats “should” win the House popular vote by about 8 percentage points — just what the other metrics show.

So you’d expect Democrats to do pretty well based on the historical propensity of opposition parties to gain ground in midterm elections, especially under unpopular presidents. And Democrats are doing roughly as well as you’d expect them to according to most indicators of the national environment.

There are a couple of exceptions — indicators that are a little out of the consensus — but both of them fall on the better-for-Democrats side of the consensus. First, Democrats have done really impressively in fundraising. Their candidates have raised more in individual contributions than Republicans in 71 of the 101 districts rated as competitive by the Cook Political Report, despite the fact that about two-thirds of these districts feature Republican incumbents. That’s unusual. Most challengers significantly trail in fundraising at this point in the cycle. Meanwhile, the results of special elections have been very good for Democrats. Our model doesn’t actually use special election results in its forecasts, but they’re part of a coherent alternative narrative in which there’s upside for Democrats relative to what our forecast shows. Donating money and voting in special elections are tangible indicators of voter engagement, and it’s possible that they point toward a Democratic enthusiasm advantage that could become clearer later on in the cycle.

Theme No. 2: However, there’s some feast-or-famine risk for Democrats

It’s much to Democrats’ credit that there are so many districts in play in all corners of the country. (Based on our accounting, Democrats have fielded a nominee in all but three of the 435 congressional districts nationwide.) But if you had to pinpoint the exact districts that Democrats should hope to win to gain 23 seats and take the House majority, you’d have a pretty hard time. We have only 215 seats rated as favoring Democrats — “lean Democrat” or stronger — which is fewer than the 218 they need to take the House.

Nonetheless, Democrats are favored to win the majority if current conditions hold because they’ll have a bunch of opportunities, even as underdogs, to win those extra seats: 14 toss-up races, 19 “lean Republican” races and 53 “likely Republican” contests. Those are a lot of lottery tickets to punch, even if Democrats aren’t necessarily favored in any individual race.

But Democrats would have a problem if there’s a shift in the national climate toward Republicans, or even if there’s a relatively modest systematic polling error in the GOP’s favor on Election Day. All of the sudden, they’d lose most of the toss-up races along with some of the “lean Democrat” races — and the “lean Republican” and “likely Republican” seats would become an uphill climb at best.

The flip side to this is that if the political environment gets better for Democrats, their seat gains could pile up at an accelerating rate. There are a plethora of districts that are 10 to 20 points more Republican than the country as a whole, a lot of which were gerrymandered to be “safe” for Republican candidates — but where the gerrymanders could fail in the event of a large enough wave.

Theme No. 3: Incumbents — especially Republican incumbents — are really vulnerable

The first line of defense for a party hoping to maintain its majority is incumbency. Even if the national political climate is unfavorable, its incumbents may be popular enough in their districts to withstand the wave.

The issue for Republicans is that the incumbency advantage has been weakening over time — and it appears to be especially flimsy this year. In the 1990s, incumbents overperformed the partisan baseline of their districts by somewhere on the order of 20 percentage points. (So, for example, a district that might favor Republicans by 2 points in an open-seat race would go to the GOP by 22 points if there were a Republican incumbent running.) In more recent elections, as Congress has become less and less popular, the incumbency advantage has eroded to more like 10 to 12 percentage points. And between Republicans’ anemic fundraising, GOP incumbents’ voting records — which are highly aligned with President Trump’s positions, even in purple districts — and reasonably good district-by-district polling for Democratic challengers, our model is projecting only about a 6-point advantage for GOP incumbents this year. Plus, a lot of Republican incumbents have retired.

Our forecast also shows a relatively narrow advantage for Democratic incumbents. But Democratic incumbents have little exposure in the House: Any Democratic representative who was strong enough to survive the GOP waves in both 2010 and 2014 probably won’t have any problems this year. (It’s a entirely different story in the Senate, where there are lots of vulnerable Democratic incumbents who were last re-elected in the strong Democratic year of 2012.)

Theme No. 4: Potential Democratic gains are broad-based, across all regions of the country

One factor helping Trump in 2016 was that he really needed to beat his polls in only one part of the country, the Midwest, to defeat Hillary Clinton in the Electoral College. (Outside of the Midwest, the polls were reasonably accurate and even underestimated Clinton in some states.) By contrast, Republicans are facing a multi-front assault in the House this year:

• In the Northeast, they have a lot of exposure in New York and New Jersey, which were once bastions of moderate Republicanism but which have become increasingly inhospitable to this brand of politics — and in Pennsylvania, where court-ordered redistricting resulted in a bad map for Republicans and where a lot of GOP incumbents have retired.
• In the South, they face pressure because of demographic change in states such as Georgia and Virginia — and increasingly in Texas.
• In the Midwest, there’s the risk of reversion to the mean with Trump off the ballot, especially as the GOP coalition in these states has come to rely on voters without a college degree who don’t always participate in midterm elections.
• And in the West, there are 14 Republican-controlled seats in California and another four in Washington that look increasingly out of place as the Pacific Coast becomes a somewhat literal “blue wall.”

As it happens, projected Democratic gains are almost evenly distributed between the four Census Bureau regions: The Classic version of our model projects them to gain nine seats in the Midwest, nine in the South, nine in the Northeast and eight in the West. Note that Democrats could completely flop in any one of these regions and yet still (just barely) win enough seats to take the House.

Theme No. 5: Democrats need to win the popular vote by a fairly wide margin

The Classic version of our model gives Democrats a near certainty (about a 98 percent chance) of winning more votes than the GOP in the race for the House — but “only” a 3 in 4 chance of winning the majority of seats. This discrepancy between votes and seats reflects a combination of gerrymandering, voter self-sorting and incumbency, all of which favor Republicans to some degree. Thus, in the Classic version of our forecast, Democrats would need to win the popular vote by about 5 percentage points in order to become favorites to win the majority of seats in the House. And in the Lite and Deluxe versions, the break-even point is closer to a 6-point popular-vote win.

Nonetheless, these margins aren’t as bad for Democrats as they might be. At earlier points in the cycle, Democrats had appeared to need more like a 7- to 8-point advantage in the national popular vote to be favored to claim the majority of seats. Since then, the Republican edge has been eroded by retirements, by Pennsylvania’s redistricting and by the relatively weak GOP incumbency advantage (see Theme No. 3). All of this might seem like splitting hairs, but because so many indicators (see Theme No. 1) point toward Democrats winning the popular vote by a margin of something like 7 or 8 percentage points, these subtle differences are important.

I’ll be on vacation next week — excuse me, I’ll be investigating ground-level conditions in Maine’s 2nd Congressional District — but we’re going to be returning to these themes again and again between now and Nov. 6, so let’s call it a day. As a bonus, though, here’s a table put together by my colleague Julia Wolfe showing what our Classic forecast thinks of the race in every district in the country.