# Electoral College Margin Right on the Mark

### President’s Electoral Vote Share Follows Historical Trend

Political scientists use the term “swing ratio” to measure how a party’s share of the seats in a legislative body like the U.S. House of Representatives varies with changes in the share of the popular vote cast.  The swing ratio is often calculated from the best-fit line using a simple linear regression of shares of seats against shares of votes.  I have applied the same concept to the Electoral College as represented in the chart above, plotting the winner’s two-party* share of the Electoral College in each election since 1960 against the corresponding share of the popular vote. The best-fit line between the share of the Electoral College vote and the share of the popular vote for President is

% Two-Party Share of Electoral College = 3.76 x (% Two-Party Share of Popular Vote) – 132

The swing ratio is the slope coefficient, 3.76.  For each one-percent increase in the popular vote share, the winner’s share of the Electoral College grows by nearly four percent.

President Obama’s performance on Tuesday falls right in line with recent historical expectations.  He won 51.2% of the two-party vote on Tuesday earning him 61.7% of the votes in the Electoral College (including Florida).  That figure is just a bit higher than the 60.5% predicted by the equation above.

While the President won both this year’s popular and electoral votes, he did so by reduced margins. Despite those losses, the President’s share of the Electoral College was greater than that won by either Jimmy Carter or George W. Bush, whose shares of the popular vote were quite similar to the President’s.  In 2004 President Bush won an identical 51.2% of the popular vote but could garner only 53.3% of the Electoral College, some nine points behind President Obama’s showing on Tuesday.  These comparisons reinforce Nate Silver’s warnings yesterday about problems for the Republicans in the Electoral College over the years ahead.  While both Ronald Reagan and his successor George H.W. Bush won larger than expected majorities in the Electoral College, “Bush 43” underperformed in both his elections.

President Obama is the only President since 1960 to see his margin of victory fall when he ran for a second term.  All three Republicans who served two terms over this period — George W. Bush, Ronald Reagan, and Richard Nixon — received a higher percentage of both the popular vote and the electoral vote when they ran a second time.  Richard Nixon in particular skyrocketed from his narrow 1968 victory over Hubert Humphrey and George Wallace to a landslide against George McGovern four years later.  A nearly comparable improvement was recorded by Lyndon Johnson in 1964 if compared to the results for his predecessor John Kennedy four years earlier.  Ronald Reagan’s dramatic success in the Electoral College in 1980 left him little room for improvement four years later, but he still managed to increase his margin in the Electoral College from 90.9% to 97.6% in 1984.

I’ve also marked the point on the graph corresponding to the minimum share of the popular vote required to secure a victory in the Electoral College.  That figure is 48.4%.  Based on the contemporary relationship depicted above, the possibility of a President winning a minority of the popular vote but a majority in the Electoral College remains a distinct possibility.

*I follow the standard practice of measuring shares of the two-party vote to remove the effects of third parties.  This approach generally does not appear to distort the results.  Three of the four elections with substantial third-party challenges, George Wallace in 1968 and Ross Perot in both 1992 and 1996, fit the overall relationship quite closely.  In 1980 though, Ronald Reagan’s extraordinary success in winning over 90% of the Electoral College may have been helped by John Anderson’s candidacy.  Reagan won eighteen states by a margin smaller than Anderson’s share of the vote, and in ten of those states the margin of victory was less than half Anderson’s vote. Excluding the 1980 election flattens the line by a small amount as one would expect after a large positive “outlier” is excluded.  The line in this case crosses through the points representing President Nixon’s totals in 1972 and President Obama’s in 2008.  The basic relationship remains fundamentally unchanged. (Return)