As of Friday at 4 p.m. EDT, bettors are less bullish on Trump than they were earlier in the week. The betting – which legally can't be done in the U.S. – put the likelihood of Trump winning the election at 51% and Harris' at 48%. Just ahead of the convention, bettors placed Harris' odds of winning as high as 54%.
How Harris' and Trump's odds of winning the election changed during their convention weeks
As part of his academic research, Harry Crane , a Rutgers University statistics professor, has studied election outcomes since 2016 and compared those to the predictions of polling and betting markets like Polymarket.
During the Democratic convention week, polling tracked by Real Clear Politics showed Harris' national average continuing to rise while Trump's inched down. The online bettors on Polymarket and Betfair , the world's largest online betting exchange, sent those markets in opposite directions.
How presidential election polls compare to election odds
Why are Kamala Harris' odds tumbling but presidential polls rising?
So what are the bettors saying? "The market price at any given time reflects the collective opinion of the market based on the information that's available," Crane said and speculated at what bettors might be considering:
◾ Scrutiny of Harris' proposals: Harris outlined her economic plans on Aug. 16, and bettors may foresee potential pushback from voters in key areas. "The people who are driving these markets have quite a bit of science behind it," Crane said. "They might know the demographics of a state like Pennsylvania – down to where they understand the sliver of the population that's really going to decide the election."
◾ Less favorable polling: While the Harris campaign continues to see incremental gains in national polls, Trump polled 2 percentage points better in the most recent poll of voters in Arizona , a battleground state.
Crane said similar speculation about Trump's potential opponent likely drove his numbers down during the Republican convention. Crane said bettors probably saw Harris as a stronger Democratic candidate, which pushed down Trump's odds of winning as it looked more likely that Joe Biden would drop out .
Is there a post-convention bump?
Crane said bettors likely anticipate some bump in the polls following the conventions, so they discount such changes in their analyses. But significant election bumps also appear to be a thing of the past.
How much presidential candidates' polling improved following conventions
Since 1964 presidential candidates have had about a 5 percentage point convention bounce in polling, according to research by The American Presidency Project . That hasn't happened since 2008 when John McCain and Barack Obama watched their numbers jump 6 and 4 points, respectively.
Up next: What odds will bettors give Trump's and Harris' campaigns following their first debate scheduled for Sept. 10.
How accurate have election odds been in past presidential elections?
The betting favorite has only lost twice since 1866 , according to the Conversation, a nonprofit news organization.
The two upsets came in 1948 when Democrat Harry Truman beat eight-to-one odds to defeat Republican Thomas Dewey, and in 2016 when Trump overcame seven-to-two odds to beat Hillary Clinton.
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