Here's the current equation. It's linear:
The good news is that with even a smattering of mathematical ability, some yahoo can make the cover of Newsweek. With the following data the guy derived a linear model, which any statistical package can do.
VOTE PARTY PERSON DURATION WAR GROWTH INFLATION GOODNEWS
1880 50.220 -1 0 1.75 0 3.879 1.974 9
1884 49.846 -1 0 2.00 0 1.589 1.055 2
1888 50.414 1 1 0.00 0 -5.553 0.604 3
1892 48.268 -1 1 0.00 0 2.763 2.274 7
1896 47.760 1 0 0.00 0 -10.024 3.410 6
1900 53.171 -1 1 0.00 0 -1.425 2.548 7
1904 60.006 -1 0 1.00 0 -2.421 1.442 5
1908 54.483 -1 0 1.25 0 -6.281 1.879 8
1912 54.708 -1 1 1.50 0 4.164 2.172 8
1916 51.682 1 1 0.00 0 2.229 4.252 3
1920 36.119 1 0 1.00 1 -11.463 0.000 0
1924 58.244 -1 1 0.00 0 -3.872 5.161 10
1928 58.820 -1 0 1.00 0 4.623 0.183 7
1932 40.841 -1 1 1.25 0 -14.557 7.160 4
1936 62.458 1 1 0.00 0 11.677 2.454 9
1940 54.999 1 1 1.00 0 3.611 0.055 8
1944 53.774 1 1 1.25 1 4.433 0.000 0
1948 52.370 1 1 1.50 1 2.858 0.000 0
1952 44.595 1 0 1.75 0 0.840 2.316 6
1956 57.764 -1 1 0.00 0 -1.394 1.930 5
1960 49.913 -1 0 1.00 0 0.417 1.963 5
1964 61.344 1 1 0.00 0 5.109 1.267 10
1968 49.596 1 0 1.00 0 5.070 3.156 7
1972 61.789 -1 1 0.00 0 6.125 4.813 4
1976 48.948 -1 0 1.00 0 4.026 7.579 4
1980 44.697 1 1 0.00 0 -3.594 7.926 5
1984 59.170 -1 1 0.00 0 5.568 5.286 8
1988 53.902 -1 0 1.00 0 2.261 3.001 4
1992 46.545 -1 1 1.25 0 2.223 3.333 2
1996 54.736 1 1 0.00 0 2.712 2.146 4
2000 50.265 1 0 1.00 0 1.603 1.679 7
Here's another article about such models.