Betting markets were shifting significantly in favor of President Trump in the first hour of the presidential debate, as shares predicting Governor Newsom winning the Democratic nomination climb.

Early in the night, betting odds on Smarkets, a website that allows bettors to bet on the outcome of political events, gave Trump a 55 percent chance of winning the election and Mr. Biden a 35 percent chance. About half way through the debate Trump’s odds rose to 58 percent and Mr. Biden’s chances fell to 19 percent.

At the same time, shares predicting that California’s governor, Gavin Newsom, would clinch the nomination and win the general election climbed to 12 percent.

On another betting site, PredictIt, shares predicting that Mr. Biden will win the nomination fell significantly during the debate, dropping to 60 cents from 86 cents on the platform while shares predicting Mr. Newsom wins the nomination rose to 21 cents from 8 cents.

Election Betting Odds, a site maintained by two betting analysts, Maxim Lott and John Stossel, that tracks the betting odds across multiple political betting sites found that the negative reaction to Mr. Biden was universal across the markets.

On average, bettors cut 15 points off of Mr. Biden’s chance of winning to 21 percent, while Mr. Trump’s odds of winning climbed by 4 points to 60 percent.

Meanwhile, the betting odds predicting that Mr. Newsom, Vice President Harris, and former first lady Michelle Obama would win the presidency rose during the debate. Notably, odds predicting attorney Robert Kennedy Jr. wins were stagnant.

As most debate watchers seemed to think that Mr. Biden’s performance was significantly hurting his chances of winning the election, Republican consultant Frank Luntz, famous for his work with focus groups, claimed that the debate was good for Mr. Biden.

“My group of undecided voters are moving away from Trump as he gets more and more vicious towards Biden,” Mr. Luntz said in a tweet. “They complain that this sounds like a senior citizen bingo hall argument.”