Many of the individual race forecasts in the FiveThirtyEight Senate model, which launched on Wednesday, look pretty optimistic for Democrats. The model shows Sen. Joe Manchin in a strong position to retain his seat in West Virginia, for instance. It has Democrats as ever-so-slight favorites to win the GOP-held Senate seats in Nevada and Arizona. It thinks Democratic incumbents like Missouri’s Claire McCaskill and North Dakota’s Heidi Heitkamp could close well down the stretch. It even gives Democrat Beto O’Rourke a credible shot in Texas — although it has Sen. Ted Cruz as the favorite in the race.
But despite that, the model has Democrats as reasonably clear underdogs to take control of the Senate. Even though it’s more optimistic than the consensus about Democrats’ chances in several individual races — and even though the model is generated by the same program that gives Democrats around a 5 in 6 chance of winning the House — it nevertheless says Republicans have somewhere between a 2 in 3 and 7 in 10 chance to hold the Senate, depending on which version of our model you look at.
This isn’t any sort of paradox: The Senate map is simply very, very daunting for Democrats. In fact, it’s about as unfavorable a map as any party has faced in the Senate, ever. Democrats have 26 seats up for election in November; Republicans have just nine. Moreover, 10 of those 26 seats are in states that President Trump won in 2016, including five states (West Virginia, North Dakota, Indiana, Montana, Missouri) where he won overwhelmingly.
Just how uphill a task is that for Democrats? We currently forecast Democrats to win the popular vote for the U.S. House by 8 to 9 percentage points (similar to their advantage on the generic congressional ballot) — a margin that by almost any definition would qualify as a “wave election.” As a point of comparison, Republicans won the House popular vote by 7 percentage points in 1994 and in 2010, and Democrats won it by 8 points in 2006, all of which are usually considered wave years. But our model thinks that even an 8- or 9-point advantage would probably not be enough for Democrats to win the Senate. Instead, they would need around an 11-point advantage in the House popular vote before becoming favorites to claim the Senate, our model estimates.
But just as Republicans are far from doomed in the House, they are far from safe in the Senate. Democrats need to gain only a net of two seats to take control of the Senate, and they have five plausible opportunities: Arizona, Nevada, Tennessee, Texas and (most debatably) the Mississippi special election, which involves a nonpartisan blanket primary on Nov. 6 with a potential runoff three weeks later. Meanwhile, Republicans have three very good opportunities to pick off Democratic incumbents — those are McCaskill in Missouri, Heitkamp in North Dakota and (surprisingly) Bill Nelson in Florida — and there are several other states where they’re still in the running, such as Indiana. But no one of those races is a sure thing for Republicans. In fact, the Classic and Deluxe versions of our model have Republicans as slight underdogs in all of the Democratic-held seats, although the polling-driven Lite version of the forecast has them favored in North Dakota and Florida.
In essence, there are two ways by which Democrats might win the Senate: a macro path and a micro path.
First, the macro path. As I mentioned, Democrats would become favorites to take the Senate if they won the overall popular vote for the House by more than about 11 points. At that point, the tailwind would simply be so strong that Democrats would probably find a way to win all or almost all of the toss-up races. (Keep in mind that Senate races are not truly independent of one another; instead the outcomes are correlated to a meaningful extent.) We wouldn’t bet on Democrats winning the House popular vote by 11 points, which would be an overwhelming margin — the most lopsided margin for either party since 1982. But considering that they’re already ahead by 8 or 9 points right now, it can hardly be ruled out.
Or, the Democrats could win by means of the micro path and just have the coin come up heads in a lot of the toss-up races, even if the overall political environment isn’t any better for them than we’re currently projecting. In our Classic forecast, there are 11 seats that each party has at least a 10 percent chance of winning: Arizona, Florida, Indiana, Missouri, Mississippi, Montana, North Dakota, Nevada, Tennessee, Texas and West Virginia. Democrats need to win eight of those 11 to take the Senate. If each of these races were truly a coin flip — meaning, a 50-50 proposition where the outcomes were independent from one another — then Democrats would have to do the equivalent of coming up with eight heads in 11 tries, the chances of which are about 11 percent.
Overall, the macro path is probably the easier one for Democrats. That is to say, if you woke me up on Nov. 7 and told me that the Democrats had won the Senate, I’d guess it had been because the overall political environment had been even better for them than we’d expected — and not because they’d navigated their way through the thicket of individual races.
With that said, almost all the Senate races are fascinating on an individual level. They generally feature high-quality candidates; Democrats are competing almost everywhere they can compete, and Republicans have mostly avoided nominating the Todd Akins and Christine O’Donnells of the world, instead having chosen more traditional nominees. In quite a few races, moreover, there’s a clash between polling and “fundamentals” — non-polling factors like fundraising and a state’s past voting history. The model is skeptical that Democrats have truly made Tennessee a toss-up, for instance, even though that’s what polls show there. On the flip side, it’s surprised that Nelson, McCaskill and Heitkamp aren’t polling better given that parties rarely lose many of their own seats in wave elections. (We’ll go into more detail on these themes in an upcoming Election Update.)
Finally, a couple of notes on how the Senate and the House forecasts interact with each other. As we explain in our methodology post, the forecasts are literally generated by the same model, so data from the Senate forecasts can affect the House numbers and vice versa.
And one scenario we can almost rule out is Democrats winning the Senate but failing to take the House. There’s less than a 2 percent chance of that, according to our model, simply because the Senate is such a heavy lift for Democrats that they’ll almost certainly have won the House by the time they get there.
How our House and Senate forecasts interact
According to FiveThirtyEight’s 2018 midterm forecasts, as of Sept. 12
|Scenario||Probability of scenario occurring|
By the same measure, the election is in a relatively precarious balance. Sure, right now, the Democrats are favored to take the House and the Republicans to hold the Senate. But even a relatively modest shift in the Democrats’ direction would make them favorites to take the Senate as well, and likewise for Republicans and the House. In fact, we’d bet against both of our forecasts being right simultaneously! There’s a greater than 50 percent chance that either Republicans win the House or Democrats win the Senate1 by the time we get to Election Day.
Check out all the polls we’ve been collecting ahead of the 2018 midterms.