FiveThirtyEight

Our congressional forecasts reflect a blend of several different methods of prediction. But for the most part, those methods tell a fairly consistent story. In the House, for instance, district-by-district polls, the generic congressional ballot and historical trends in midterm elections all point toward Democrats winning the national popular vote by somewhere in the range of 8 to 10 percentage points, which would very probably be enough for them to take control of the chamber. So do what we call the “fundamentals,” non-polling indicators that have empirically been useful predictors of races for Congress, such as fundraising totals, past margins of victory and several factors related to incumbency.

Likewise in the Senate, the different versions of our model, which blend these methods together in different ways, tell a similar overall story to one another. It’s a reasonably happy story for Republicans because the Senate map, which consists overwhelmingly of Democratic-held seats, is highly favorable for the GOP this cycle. The polling-driven Lite version of our Senate forecast has the GOP finishing with 51.3 Senate seats and having about a 5 in 7 chance (more precisely, 71 percent) of keeping control of the chamber. The Classic version, which incorporates fundamentals, has them with 50.8 seats and about a 2 in 3 chance (68 percent) of maintaining control instead. If you go down to the decimal point, the Classic forecast is ever-so-slightly better for Democrats than the Lite forecast — which implies that the fundamentals are ever-so-slightly better for them than polls — but it isn’t a big overall difference.

The word “overall” is doing a lot of work in the previous sentence, however, because while the top-line prognosis on who might control the Senate is similar in all three versions of our model, the forecasts differ quite a bit from race to race. In particular:

In this article, I’m going to focus on the first category only: races featuring Democratic incumbents. We’ll cover the other two categories in an upcoming piece.

The ‘fundamentals’ say it should be really hard for Democratic incumbents to lose

(Warning: Sports analogy ahead.) In soccer, the most obvious advantage of having a man advantage after an opposing player has been sent off on a red card is that it’s much easier to score goals. But what’s nearly as important is that it becomes very difficult for the other team to score a goal; defense becomes a cinch.

So it also goes in congressional races when your party is having a wave election: You’ll win plenty of your opponent’s seats, but — not to be overlooked — you’ll usually also lose almost none of your own seats. The GOP didn’t lose any of its own Senate seats in 1994, 2010 or 2014, for example. Democrats didn’t lose any of their own seats in 2006 or 2008. Between these five election cycles, the “waving” party went undefeated in defending its Senate seats.



This year, however, we have a seeming contradiction: The polls are pointing toward a wave in the House, with an average projected gain of 35 to 40 seats for Democrats and a popular vote win of 8 to 10 points. And yet, Democrats are at risk of losing several of their own Senate seats, which could offset any gains they may make among GOP-held seats and make it much harder for Democrats to take control of the Senate.

One way out of the dilemma for Democrats is if their incumbents in the Senate aren’t in quite as much trouble as the polls show — and our fundamentals calculation suggests that could be the case. Below is a comparison of polls and fundamentals in each seat held be an elected Democratic incumbent. Note that elected incumbent excludes appointed incumbents such as Tina Smith in the Minnesota special election; we’ll deal with her race in the next installment.

Most Democratic incumbents are doing worse than the ‘fundamentals’ would predict

Forecasted margin of victory for Democratic senators who are running for re-election, according to FiveThirtyEight’s fundamentals and adjusted polls

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*King and Sanders are independents who caucus with the Democrats.
The Minnesota special election is not listed because Tina Smith is an appointed rather than an elected incumbent and our model treats races with appointed incumbents as open-seat races. California is not listed because the race features two Democrats and no Republicans. There is no polling of the Senate races in Maryland, Vermont and Hawaii.
Adjusted polling as of Sept. 17; fundamentals as of Sept. 16.

A handful of Democrats, such as West Virginia’s Joe Manchin, Ohio’s Sherrod Brown and Pennsylvania’s Bob Casey, are running slightly ahead of their fundamentals, but most Democrats are underperforming them. Florida, North Dakota and Missouri, where polls show near toss-ups in races that the fundamentals suggest should be Democratic-leaning, are the most important cases. Montana’s Jon Tester and Indiana’s Joe Donnelly, although they lead in most polls, also fall into this category, as fundamentals suggest they should have a slightly clearer advantage. Polls also show New Jersey’s Bob Menendez in a somewhat competitive race, when fundamentals imply Republicans should have no business competing in New Jersey in such a Democratic-leaning year — even accounting for Menendez’s corruption scandal.

Before we go further, it might be helpful to break fundraising out from the other factors we include in the fundamentals. Why? It isn’t necessarily because fundraising is the most important or predictive factor, although, it’s one of the most predictive ones. Rather, it’s because we tend to think of fundamentals as background conditions, whereas fundraising is in part a reaction to those conditions. Voter enthusiasm, which by most indications is on Democrats’ side this year, can translate to higher fundraising totals, for instance.

Democrats have a fundraising advantage in every single Senate race with a Democratic incumbent. (Our evaluation of fundraising is based on individual contributions only, not money from PACs or parties or self-financing from the candidates.) This isn’t a huge surprise — the incumbent candidate usually raises more than the challenger. But the Democratic fundraising edge is quite emphatic, with the median Democratic incumbent in the Senate having almost a 6-1 fundraising advantage over his or her Republican opponent. This makes a pretty big difference. As you can see in the next chart — which breaks out how much fundraising contributes to the fundamentals calculation in each race — there are several races where the Democratic incumbent’s edge would otherwise be fairly marginal, but where fundraising tips the scales toward them.

Democrats’ fundraising advantage helps buoy their lead

Forecasted margin of victory for Democratic senators who are running for re-election, according to various aspects of FiveThirtyEight’s fundamentals

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*King and Sanders are independents who caucus with the Democrats.
The Minnesota special election is not listed because Tina Smith is an appointed rather than an elected incumbent and our model treats races with appointed incumbents as open-seat races. California is not listed because the race features two Democrats and no Republicans. There is no polling of the Senate races in Maryland, Vermont or Hawaii.
Adjusted polling as of Sept. 17; fundamentals as of Sept. 16.

It’s also worth describing how our model “thinks” about incumbency — and how the incumbency advantage is a bit more complex than people might assume. For example, were it not for an overwhelming fundraising advantage over his Republican opponent Rep. Jim Renacci, the fundamentals model would actually have Ohio’s Brown as being more vulnerable than West Virginia’s Manchin.

How can that be? Isn’t West Virginia much redder than Ohio? Well, sure. (Although Ohio is getting pretty red too.)

But the most important fact about incumbents is that incumbents, by definition, won their previous election. So it can help to look at the incumbent’s previous margin of victory for clues about his or her strength. For instance, West Virginia is much redder than Ohio now, but that was also true in 2012 — and in 2012, Manchin won his race by 24 points, while Brown won his by just 6. That’s a hint that the incumbency advantage isn’t one-size-fits-all and that there are some characteristics — either relating to Manchin himself or to his state, West Virginia — that made him a stronger incumbent than Brown.

One clear trend in the data, for instance, is that incumbents from smaller-population states such as West Virginia tend to be harder to beat than those from more populous ones such as Ohio. Look up the historical rosters of senators from Hawaii or Delaware, for instance, and you’ll tend to see the same incumbents getting re-elected every six years until they finally retire or pass away; less so in larger states such as Florida. I wasn’t surprised when we discovered this trend in the data; there are several fairly straightforward reasons why you’d expect it to be the case.

Another pattern is that more demographically and politically idiosyncratic states tend to be associated with larger incumbency advantages. We measure each state’s similarity to other states by means of its CANTOR score, which evaluates how comparable it is to other states based on a variety of demographic, political and geographic variables. We found that states that have fewer good comparables — say, West Virginia or Vermont, or even California — tend to re-elect their incumbents more often than states whose demographics more closely reflect those of the country as a whole, such as Ohio or Missouri. This also ought to be fairly intuitive; it’s easier for an incumbent to be a uniquely good fit for his or her state if that state is more unique.

Manchin also has a considerably more moderate voting record than Brown, another factor that our model considers. He’s voted with the Democratic Party only 63 percent of the time over the past three Congresses, according to our version of a party unity score, as compared with 89 percent of the time for Brown. Our model finds that incumbents who break ranks with their parties more often tend to do better than those who always vote the party line, other factors held equal.

With that additional background, let me quickly go through the model’s logic on some of the key races with Democratic incumbents.

One final, important note: While several of the Democratic incumbents, such as Heitkamp and Menendez, are underperforming where they “should” be in the polls, our model will give them only so much time to catch up to its expectations. As time passes and as more and more polls come in, the effect of the fundamentals will diminish in our forecast — to the point where they really only serve as a tiebreaker in Senate races that are otherwise too close to call. We’ll cover GOP incumbents and open-seat races, where the fundamentals calculation sometimes helps Republicans, in another article soon.


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