Democrat candidate Vice President Kamala Harris' odds at presidency, on the contrary, declined from 41.3% to 38.3%.
Polymarket's estimate of total bets on the US presidential election is exceeding $3.2 billion.
US Election Day is set for November 5, and at least 186 million Americans aged 18 or over can vote for the 47th US president.
The two leading contenders are former President Donald Trump (Republican) and Vice President Kamala Harris (Democrat). Apart from Trump and Harris, an independent candidate and one each from the Libertarian and the Green Party will compete for office.