As you can see, the trend is a powerful one: Historically, the electorate has swung almost 10 points toward the party out of power. Even in modern, post–Civil War history, the swing in post-incumbent elections averages 6.68 points in the two-party vote and 5.95 points in the popular vote. And remember, 6.68 points is only one side of the coin, since those points go to the other side — it’s enough to close a gap of more than 13 points. Even including the post-incumbent elections in which another incumbent was running, it’s still an average swing of more than 8 points, and more than five points in modern times.
In the two-party vote, the only true post-incumbent election to buck the trend was 1816. In 1812, President James Madison and the opposition Federalists bet everything on their stances on the War of 1812. The closely divided election of that year was held after the outbreak of war but long before its outcome was known. The war ended in a draw, but at its conclusion, it was hugely popular, seen widely as a vindication of American independence from England. The Federalists were permanently discredited. After James Monroe, who had been secretary of state and secretary of war during the war, crushed Rufus King and the Federalists to succeed Madison in 1816, the Federalists never fielded another presidential candidate.
The only election since with less than a 4-point swing was 1868, when the party in power (the Republican party) was running a national hero (Ulysses S. Grant) and the sitting incumbent (Andrew Johnson) was actually a Democrat who had been impeached and almost removed from office by the Republican Congress. As a result, Grant could run as the de facto opposition. Oh, and the Democrats’ geographic base was under military occupation by Grant, after having gone to war against the national government. The Republicans still dropped 2.38 points in the two-party vote.
So, for 200 years, every opposing candidate in a true post-incumbent election has pulled the two-party vote at least 2 points in their direction. Every election, that is, until this one. Trump benefited from a shift of just 0.85 points in the two-party vote, the worst since Rufus King in 1816. Even FDR’s opponents in 1940 and 1944 did better. A candidate with nothing but the historical wind at his back would have fared far better than Trump. Only his singular underperformance of the historic trend kept this race even close.