Voters' doubts about President Biden’s fitness to serve in the Oval Office have grown over the past few months, according to a new poll released Monday amid the fallout from his shaky debate performance last week.

The latest Harvard CAPS-Harris survey, released exclusively to The Hill, found that 66 percent of respondents said they “have doubts” about Biden’s “fitness," an increase from when the poll was last conducted in May, when 54 percent of respondents said they had doubts. Meanwhile, 34 percent said he is “mentally fit,” and 74 percent said Biden is “showing he is too old” to be president. Only 26 percent said he is “showing he is fit” to be president. 

The poll found that 61 percent of voters said “electing a president who raises questions about age, failing memory, or lapsed concentration” is “dangerous for the country.” Thirty-nine percent of voters said “the fears are overblown politically." 

“Seventy percent of voters have already decided who they will back in November,” said Mark Penn, the co-director of the Harvard CAPS-Harris Poll. 

“We saw a sharp jump in concern over Biden’s age and his fitness for another term — including nearly half of Democrats,” he said. 

The findings come just days after the first presidential debate of the cycle between Biden and Trump. Despite Trump’s critics pointing out inaccuracies and falsehoods he voiced during his debate performance, Biden’s shaky performance has received the brunt of the criticism. The president appeared to stumble and struggle to form coherent sentences at times during the debate. The poll was conducted after the debate, June 28-30. 

Fifty-two percent of voters said they believed Trump had the best debate performance, while 22 percent said the same about Biden. Another 26 percent said they were undecided. 

However, the horse race between Biden and Trump essentially remains unchanged after the debate. When undecided voters are included, Trump leads Biden 49 percent support to 43 percent. In May, the poll found Trump leading Biden 47 percent to 41 percent. 

In a head-to-head without undecided voters included, the latest poll shows Trump leading Biden 52 percent support to 48 percent. In May, the poll found Trump leading Biden 53 to 47 percent. 

The poll was conducted Jan. 28-30 among 2,090 registered voters by the Harris Poll and HarrisX. Results were weighted for age within gender, region, race/ethnicity, marital status, household size, income, employment, education, political party, and political ideology where necessary to align them with their actual proportions in the population.