The Seats-in-Trouble Forecasts of the 2018 Midterm Congressional Elections


The Seats-in-Trouble model of party seat change in national congressional elections (both on-year and midterms) is a hybrid election forecasting model. It combines the insights and comprehensive assessments of expert election analysts examining in depth the conditions of individual House and Senate contests with a rigorous statistical analysis of historical aggregate data of partisan seat change.

From seats exposed to seats in trouble

The model essentially builds on the seat exposure idea explored by Bruce Oppenheimer, James Stimson, and Richard Waterman and similar notions examined by others who recognized the simple facts that you can’t lose what you don’t have and you are more likely to lose a lot if you have a lot to lose. Taking it to the next level, if you have a lot of vulnerable seats, chances are greater you’ll lose a lot of seats. The Seats-in-Trouble forecasting equation built from these observations was first used in the 2010 House elections. It has since been modified, tweaked, and extended to Senate elections. The accuracy of its forecasts in its various incarnations has been quite good until 2016, when substantial expected Democratic seat gains failed to materialize. As a result, the model’s standard of what constitutes a “seat in trouble” has now been tightened.

From individual ratings to an aggregate index

The raw material of seats-in-trouble equation is the pre-election competitiveness ratings of individual district or state races determined by The Cook Political Report since the mid-1980s.[1] Races are rated in one of eight categories: solid, likely, or leaning to the Democrats, a Democratic toss-up seat, and the same four levels of competitiveness on the Republican side. The seats-in-trouble index aggregates these individual district or state (in application to the Senate) ratings to a national measure. The revised index now counts a seat as being in trouble if it is rated as a toss-up or worse for the party currently holding the seat. The index is computed as the net difference in the number of seats in trouble for each party: Democratic seats in trouble minus Republican seats in trouble. A negative relationship is expected between the number of seats in trouble for a party and the number of seats actually gained by that party.

From an aggregate index to a statistical forecast

How has the net number of a party’s seats in trouble in August of an election year translated historically into the net number of seats it won or lost in November? The seats in trouble index is plotted against seat change for Democrats in House and Senate elections in Figures 1 and 2 respectively, and the associated regression equations are presented in Table 1.

Figure 1: Democratic seat change in the House by seats in trouble, 1984-2016

Note: Ratings around August were not made in 1986 and 1990 and those elections, therefore, are not included.

Figure 2: Democratic seat change in the Senate by seats in trouble, 1988-2016

Note: Ratings around August were not made in 1990 and are, therefore, not included.

Table 1: The Seats-in-Trouble forecasting equations of Democratic Party congressional seat change

Notes: N=15 for House, 14 for Senate. **p<.01, one-tailed. *p<.05, one-tailed. Standard errors are in parentheses. House estimates are based on elections from 1984, 1988, and from 1992 to 2016. Data was unavailable for the Senate in 1984. The seats in trouble count is from The Cook Political Report in or around mid-August of the election year. The 2018 reading was of The Cook Political Report on Aug. 17, 2018.

The House forecast

As Figure 1 and the table indicate, the index and actual seat changes are strongly related in House elections and the conversion of vulnerable seats to lost seats is greater than one in absolute value. If there are strong partisan short-term forces developing in an election, August ratings pick up their early signs, but not their full brunt as they build to November. In concrete terms, for every two “toss-up or worse” seats a party has in August compared to its opposition, it should expect to lose three seats in November.

The Cook Political Report in mid-August 2018 rated three House seats held by Democrats and 37 seats held by Republicans as toss-ups or worse. With a toss-up or worse difference of -34 for the Democrats, they are likely to gain about 44 seats. If this comes to fruition, the next House will have a Democratic majority with about 238 Democrats and 197 Republicans. Based on both in sample and out of sample errors, the odds are very high that the Democrats will emerge from the midterm with control of the House.

The Senate forecast

The association between Senate seats rated in trouble in August and actual seat change is also strong, though not quite the level of that in the House. The conversion of seats in trouble to Senate seat change is nearly one-to-one.

Because Democrats are defending a much larger number of Senate seats than Republicans this year, there are many more Democrats seats that could potentially be in trouble. As of mid-August, Cook rated five of the 26 Senate seats defended by Democrats and three of the nine Senate seats held by Republicans as toss-ups or worse. With Democrats having two more Senate seats in trouble than Republicans, we should expect Republicans to pick up two more seats, leaving the next Senate with 53 Republicans and 47 Democrats.

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.


1. Thanks to Charlie Cook and his colleagues at The Cook Political Report for generously sharing their data.