Only 40 percent of respondents say they believe Biden - the oldest president in US history - is in good health, 46 percent think that he is mentally fit and 44 percent that he is stable, the poll results show.
The poll also revealed that only 41 percent of voters believe Biden is capable overall to lead the United States.
This represents a 29-point shift in public opinion about the status of Biden's mental and physical health since the last Morning Consult survey in October of 2020.
The poll also showed that 44 percent of respondents approve of Biden's performance as president. On an issue-by-issue breakdown, poll results indicated that a mere 32 percent of respondents agree with Biden's handling of the illegal immigration problem and 38 percent of his handling of foreign policy.
Regarding Biden's personal qualities, 34 percent of respondents agreed that the president is energetic, 37 percent felt he is a clear communicator, and 39 percent found him to be a strong leader.
Biden will celebrate his 79th birthday on Saturday.