Yale economist Ray Fair is one of the pioneers of modernday election forecasting (Fair, 1978). His presidential vote equation, which was first used to predict the 1980 election, is still applied today, although, not surprisingly, it underwent several revisions in the nearly 40 years since its first publication. Basically, the model is built on four major assumptions or theories of voting:
 Incumbent presidents running have an advantage.
 Voters like change. Therefore, parties in office for two or more consecutive terms have a disadvantage.
 There is a slight but persistent bias favoring the Republican Party.
 The state of the economy affects the incumbent party vote.
Presidential vote equation
The presidential vote equation predicts the Democratic twoparty popular vote. It is formulated as (Fair, 2009; 2018):
V = 48.06 + 0.673 (G
*I
) – 0.721 (P
*I
) + 0.792 (Z
*I
) + 2.25 (DPER
) – 3.76 (DUR
) + 0.21 (I
) + 3.25 (WAR
)
Table 1: Overview of variables used in the Fair model  
Variable  Description  

G 
Growth rate of real per capita GDP in the first three quarters of the onterm election year (annual rate)  
P 
absolute value of the growth rate of the GDP deflator in the first 15 quarters of the administration (annual rate) except for 1920, 1944, and 1948, where the values are zero.  
Z 
Number of quarters in the first 15 quarters of the administration in which the growth rate of real per capita GDP is greater than 3.2 percent at an annual rate except for 1920, 1944, and 1948, where the values are zero  
I 
1 if there is a Democratic presidential incumbent at the time of the election and 1 if there is a Republican presidential incumbent  
DPER 
1 if a Democratic presidential incumbent is running again, 1 if a Republican presidential incumbent is running again, and 0 otherwise  
DUR 
0 if either party has been in the White House for one term, 1 [1] if the Democratic [Republican] party has been in the White House for two consecutive terms, 1.25 [1.25] if the Democratic [Republican] party has been in the White House for three consecutive terms, 1.50 [1.50] if the Democratic [Republican] party has been in the White House for four consecutive terms, and so on  
WAR 
1 for the elections of 1918, 1920, 1942, 1944, 1946, and 1948, and 0 otherwise  
V  Democratic share of the twoparty presidential vote 
Computer your own 2020 presidential vote forecast
Entering the variable values from Table 1 for the 2016 election, the vote equation can be written as:
V = 45.6 – 0.673 (G
) + 0.721 (P
) – 0.792 (Z
)
In the following vote equation, you can compute your own forecast of the Democratic twoparty vote by modifying the values for G
, P
, and Z
in the green highlighted textboxes:
V = 45.6 – 0.673 * + 0.721 * – 0.792 *
=
The chart below shows how the forecast from the Fair model has changed since January 1st, 2016.
References
 Fair, R. C. (2018). Presidential and congressional voteshare equations: November 2018 Update.

Fair, R. C. (2009). Presidential and Congressional Vote‐Share Equations. American Journal of Political Science, 53(1), 5572.

Fair, R. C. (1978). The effect of economic events on votes for president. The Review of Economics and Statistics, 60(2), 159173.