One of the questions we are asked the most is about why polls were wrong in the 2016 election that saw the “surprise” win for Trump. Polling averages had shown significant leads for Clinton, but the race tightened during October; polls not in the field toward the end missed the movement toward Trump, with the final popular vote margin being 2.1%. To some extent, many of the polls in the fall of 2016 weren’t wrong, as much as there was a lack of consideration for what can happen in the Electoral College when the margin is under 3%.
As we approach another close election, all scenarios should be considered if the current trajectory continues. Today’s Real Clear Politics average shows a Harris lead of 1.5% with 538’s average at 2.6%. Most of the political discussion is about Harris leading, but both of these polling averages are within the 3% range.
Looking back over previous presidential elections, the 3% margin is a trigger for the outcome to be determined by the Electoral College more than the popular vote. In cases where there was more than a 3% popular vote margin, the popular vote and the Electoral College vote agree. However, when the popular vote margin is 3% or less, the Electoral College comes into play as the determining factor.
The chart below shows the results of the 10 closest elections since 1856, sorted by the popular vote margin. Numbers in blue mean the result favored Democrats, and numbers in red favored Republicans. Highlighted rows denote elections where the Electoral College winner did not win the popular vote.
Starting with the 1856 election, which was the first contest between a Republican and Democrat, there have been 42 presidential elections. In that timeframe, there have been 10 elections in which the popular vote was 3% or less. Of those 10 elections, 4 were won by the candidate who did not win the popular vote: 1876, 1888, 2000, 2016.
If the current presidential polling margin continues to stay within the 3% window, there is the possibility that the popular vote winner could lose the Electoral College. Based on these historical trends, there is a 40% chance this could happen if the margin is 3% or less. This scenario should be considered as we get closer to the election.