This year’s US election, in which Democratic vice-president Kamala Harris will vie with Republican ex-president Donald Trump, is shaping up as a neck and neck race for the White House. In a Reuters/Ipsos poll published yesterday, exactly a week before Election Day, Harris held a one-percentage-point lead over Trump (with a 3% margin for error). RealClearPolling’s average of nationwide polls, meanwhile, gives Trump a 0.4-point advantage.

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This time 76 years ago, when Harry Truman went up against Thomas Dewey for the presidency, the pollsters were far more confident about the outcome of the US election. Their predictions were defied, however, in what is viewed as one of the greatest electoral upsets in American political history.

What happened in the 1948 US presidential election?

Ahead of the 1948 election, Democratic president Truman, who had stepped up from vice-president after the death of Franklin D. Roosevelt in April 1945, was not expected to remain in office. Opinion polls unanimously favored New York governor Dewey, the Republican nominee, to beat his less charismatic opponent. Per the Associated Press, indeed, Dewey was forecast to prevail over Truman by a margin of between five and 15 percentage points.

But when Americans cast their votes on November 2, 1948, Truman scored a comprehensive victory, triumphing by 303 electoral-college votes to Dewey‘s 189. In the popular vote, Truman was backed by over two million more people across the country - at a time when the US electorate was a little over half the size of today’s (95 million, compared to nearly 170 million registered voters when the 2020 presidential election took place). The shock result has been described as “polling’s biggest ever goof”.

Dewey had been so heavily tipped to land the presidency that one Republican-leaning newspaper went down in journalistic infamy when it prematurely announced him as the US’s new commander-in-chief. Before all the votes had been tallied, the Chicago Tribune printed an early edition with the banner headline: “Dewey beats Truman”. Clutching a copy of the Tribune, a smiling Truman later posed for what has become an iconic photograph.

How did the pollsters get it wrong in the 1948 US election?

A major flaw in the way the polls surveyed voters’ intentions in 1948 is that they stopped collecting data too long before the election was held. PBS places this gap at a week, while George Gallup Jr. - whose father and namesake was among the pollsters left red-faced by Truman’s surprise win - told a 1998 interview that polling was halted as much as “a few weeks” ahead of Election Day. “We had been lulled into thinking that nothing much changes in the last few weeks of the campaign,” Gallup Jr. told AP.

Significantly, PBS explains, this allowed pollsters to miss a late, sizeable shift in support away from independent candidates to Truman.

While election polling has become a more accurate science in the nearly eight decades since 1948, that doesn’t mean the forecasters haven’t suffered further ignominious failures. Most recently, Trump’s victory over Hillary Clinton in the 2016 US presidential election came despite nationwide surveys pointing to a victory for the Democratic nominee, right up to the day of the ballot.