How betting markets have shifted in October

The odds on Polymarket had Trump’s and Harris’ win probabilities tied at 49% on Oct. 3. Since then, Trump’s odds of defeating Harris jumped to their widest margin last week since July 21, the day President Joe Biden dropped out of the race. 

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The initial rise in Trump’s odds of winning corresponded in October to a slight increase in his polling in battleground states such as Pennsylvania. But some questioned the quick, significant jump in his probability of winning. They suggested a deep-pocketed player or players could be tampering with the betting markets. Polymarket officials have said they are investigating those claims.

Last week, the gap between Trump’s and Harris’ odds of winning was as wide as the Biden-Trump gap in the 2020 election. They since narrowed while battleground-state polling has held steady. Biden’s lead in polling in 2020 was nearly three times as large as it is between Trump and Harris.

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Bettors push Trump’s probability of winning higher than in previous elections

Trump’s probability of winning started the week several percentage points below the high the campaign hit on the opening day of the Republican National Convention. On July 16, his probability of defeating Biden was more than 70% on Polymarket and Betfair Exchange.

According to a 2004 paper in the Journal of Economic Perspectives, the clear betting favorite in the month before the election has lost only twice, in 1916 and 1948. The betting markets also failed to predict Trump’s win in 2016.