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Introduction: The 2018 Midterm Election Forecasts

A Commentary By James E. Campbell

Editor’s Note: In an effort to provide as broad a view as possible to readers about different methods of forecasting the 2018 midterm election, we have been featuring models from respected political scientists that aim to project the net seat change in the U.S. House of Representatives. So far, we’ve published models from Crystal Ball Senior Columnist Alan Abramowitz and from the team of Joseph Bafumi, Robert Erikson, and Christopher Wlezien. Both suggest the Democrats are favored to retake the House majority. This week, we’re featuring two more models from James Campbell and the team of Charles Tien and Michael Lewis-Beck. They too forecast a Democratic House takeover, and of a bigger size than the two previously published models. However, these forecasts also address the Senate, and they suggest Republicans are favored to retain control of the upper chamber. Those two models are described in this week’s Crystal Ball. But first, Campbell has an introductory piece explaining these models and the overall midterm environment. Explanations of all of these models, as well as Campbell’s introduction, will be appearing in PS: Political Science and Politics. We’d like to thank Campbell for once again convening this collection of forecasts, which is something he does for the Crystal Ball and PS every two years. For details about the four forecasts included in this year’s collection, click on the names above.

The Crystal Ball Editors


On Tuesday, Nov. 6, about 90 million American voters (around 40% of the voting-eligible population, give or take) will elect all 435 members of the U.S. House of Representatives and 35 members of the U.S. Senate.[1] The midterm election’s outcome will play a major role in policy-making and the politics leading up to the presidential election of 2020. Going into the 2018 elections, Republicans hold majorities in both chambers of Congress. This collection of four different models printed in the Crystal Ball offers forecasts of how the 2018 midterm congressional elections are likely to change the partisan composition of the House and the Senate.

As in previous forecasting symposiums, the forecasts in this collection are based on statistical analyses of how indicators of the pre-campaign context of elections have been associated historically with their outcomes. While each model draws on its own set of indicators, some context may be helpful. What do the history of midterm elections and the number of seats each party is defending in 2018 (the arithmetic of the election), and the political climate leading up to the fall campaign broadly indicate about 2018?

Midterm history

As is well known, the president’s party routinely loses House seats in midterms. The presidential party has lost seats in all but three (1934, 1998, and 2002) of the 29 midterms held since 1900. In the 17 midterms held since 1950, presidential party losses averaged 24 House seats. Since the mid-1980s, the mean loss has been 20 seats and a median of only 10 seats.

While midterm losses in House seats are routine, they vary greatly, and this variation has increased substantially. The standard deviation of presidential party midterm seat losses increased from a standard deviation of 17 seats (1950 to 1982) to 27 seats (1986 to 2014).   Polarized parties have diminished the number of easily flipped seats; but, in doing so, they have increased the number of seats taken when short-term conditions more strongly favor one party. House seat changes are in an era of feast or famine. Based on the history of seat changes, the real question is not which party will gain seats, but whether Democratic House seat gains will be small or large.

The congressional arithmetic of 2018

The number of seats each party holds and needs to win to achieve a majority, the congressional arithmetic of 2018, is also an important context of the election. At first glance, current party divisions would seem to favor Democrats in the Senate and Republicans in the House. However, the prospects of Democratic majorities are just the opposite: greater in the House than in the Senate. Despite their presidential candidate losing the national popular vote and losing a net six seats, Republicans won a 24-seat majority (241 Republicans to 194 Democrats) in the House in 2016. So, using the prior election as the baseline, Democrats need a pickup of 24 seats to reach 218 seats for a majority.[2] As the midterm history reviewed above indicates, a midterm loss for Republicans of this magnitude would not be especially unusual.

In the Senate, Republicans hold a slim majority (51 to 49, counting two nominal “independents” who caucus with the Democrats). Taking into account Vice President Mike Pence’s tie-breaking power, Democrats require only a two-seat gain for majority control. This seemingly small shift, however, is a tall order. Democrats are defending many more seats than Republicans this year. Of the 35 Senate seats up, Democrats hold 26 and Republicans just nine. This does not offer much room on the “high side” for Democrats or much room on the “low side” for Republicans. Democrats have to do well just to hold their current overall numbers.

The reason for this is found in the previous three elections for this class of Senate seats. In this Senate class’s last election in 2012, Democrats registered a small net gain (two seats), but this was in addition to their strong showings in the prior two elections for the class. In 2006, Democrats gained six seats, and in 2000, they gained four seats. A net gain of two seats and a majority in the Senate requires means Democrats have to win a whopping 80% (28 of 35) of this year’s Senate elections.

The political climate of 2018

That is history, but how is the current political climate steering the electorate? Despite strong economic growth (real GDP growth of 4.2% in the second quarter), federal income tax cuts, and a low and declining unemployment rate (under 4%), the political climate leading into the midterm seems to favor the Democrats. An important indicator of this is the president’s approval rating. In mid-August, President Donald Trump’s approval hovered around 40%. Is this good or bad news for the in-party?

The good news for Republicans is that then-candidate Trump’s numbers had not been high in 2016 when voters last elected a Republican House and Senate. If Republicans in Congress could survive candidate Trump in 2016, why can’t they survive President Trump in 2018? Moreover, aside from Presidents George H.W. and George W. Bush, whose approval ratings had been inflated by Desert Storm and the 9/11 terrorist attacks, President Trump’s current numbers are not much different from the ratings of other recent presidents going into their first midterm. Other than the Bushes, every president since the mid-1970s has had approval ratings in the low to mid 40s at their first midterm. As of this week, Trump’s Gallup approval rating stands at 40%.

The bad news for Republicans is that the parties of each of these four recent (non-Bush) presidents (Jimmy Carter, Ronald Reagan, Bill Clinton, and Barack Obama) suffered double-digit House seat losses in their midterms and two lost more than 50 seats (Clinton in 1994 and Obama in 2010).

The forecasts

With the history, arithmetic, and climate considered, we can now turn to the systematic estimates of how these factors are likely to come together in this year’s election. What follows are four independent congressional forecasts that inform us about what we should expect to come out of this election. Table 1 presents a summary of these congressional forecasts. Although there are differences among them, two points are common. This is likely to be a very good year for the Democrats in the House of Representatives. In fact, all four forecasts expect a Democratic House majority. In the Senate, Republicans are likely to hold their own and perhaps pick up a seat or two. So, on to the forecasts. For more details about the forecasts, see the links at the top of this piece.

Table 1: Summary of the 2018 midterm congressional election forecasts


1. Mississippi’s special nonpartisan election on Nov. 6 for its Class II seat seems likely to result in a runoff election on Nov. 27.

2. After now-Rep. Conor Lamb (D, PA-18) won a March special election, Democrats need 23 net seats to win a majority in the U.S. House.


James E. Campbell is the UB Distinguished Professor of Political Science at the University at Buffalo. His most recent book is Polarized: Making Sense of a Divided America.

See Other Political Commentary by James E. Campbell.

See Other Political Commentary by Larry Sabato.

See Other Political Commentary.

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