The table below is a list of United States presidential elections ordered by margin of victory in the Electoral College vote.
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Informal definition
The margin of victory in a U.S. presidential election, with the exception below, would be the difference between the number of electoral votes won by the candidate with at least a majority of electoral votes (currently 270 of 538) and the number received by the second place candidate. Theoretically this number would be in the range of 2 to 538. The numbers above assume that all electors vote.
The exception would occur if no candidate receives a majority of the electoral vote (this includes a tie). In that case, the House of Representatives, voting as state delegations, would choose from up to three candidates who received the most electoral votes. Thus the winner could be an initially third-place candidate who received one electoral vote.
Because the Electoral College has grown in size, the results are normalized to compensate. For example, take two elections, 1848 and 1968. In the election of 1968 Richard Nixon got a majority by 32 votes. At first glance, the election of 1848 appears closer, because Zachary Taylor got a majority by only 18 votes. But Nixon could have gotten as many as 269 votes above a majority (if he had won unanimously), while Taylor could only have gotten 145 votes above a majority. Thus, we normalize the two elections to compare them. We calculate Nixon's margin of victory by dividing the 32 by 269 to get 0.119. We do the same with Taylor, dividing 18 by 145, to get 0.124. And we find that Nixon's election was actually closer because a smaller fraction of the electors separated Nixon from a contingent election.
While the above explanation applies to modern elections, initially the process was different. Prior to the passage of the 12th Amendment, the winner of the presidential election was the person who got both a majority of electors to vote for him and who got the most number of votes, because each elector cast two presidential votes. Thus, for elections prior to 1804, if two candidates got above 50% of the electors, the margin of victory is the victorious candidate's margin over the other candidate who got above 50% of the electors. Of the four elections prior to the 12th Amendment, two involved two candidates getting above 50% of the electors: 1792 and 1800.
Mathematical definition
The margin of victory in the election is calculated as follows:
Let c be the total number of electors voting in the election. Let w be the number of electoral votes cast for the candidate with the most electoral votes, and let r be the number of votes for the runner-up.
The Constitution provides that if the candidate with the most votes does not receive a simple majority of the electors voting, the House of Representatives chooses the president. So, the margin of victory is the number of electoral votes over both the runner-up and half the electoral votes cast. For elections after the passage of the 12th Amendment, the runner-up will always have less than half of the electoral votes cast, so the absolute margin of victory will be the difference of the winner's electoral votes and half the electoral votes cast. To express this in mathematical formulae:
The minimum possible value for the margin of victory is clearly zero. The maximum possible value of the margin of victory occurs in the case in which each elector casts a vote for the winning candidate and the runner-up gets no more than half of the vote. In this case, the maximum margin of victory is c/2. In order to meaningfully compare election to election, we need that maximum margin to be constant from election to election. Thus, we divide the absolute margin of victory by c/2 to get a normalized margin of victory that ranges from 0 to 1:
Table of election results
Note that in the following table, the election of 1824 is ranked closer than the election of 1800 because the election of 1800 resulted in a two-way draw, while the election of 1824 resulted in a three-way draw.
Also note that the elections of 1789, 1792, 1796, and 1800 took place before the 12th Amendment and thus each elector had two votes (but had to vote for two separate persons).