Joe Biden holds a small lead over Donald Trump among registered voters even as the former president leads his successor on issues such as age, and mental and physical fitness in a new poll.

Forty-nine per cent of registered voters back the president, while 45 per cent support Mr Trump in a Quinnipiac University national poll released on Wednesday.

In late January, the same pollster found that 50 per cent backed Mr Biden and 44 per cent supported Mr Trump.

Among Democrats, 93 per cent support Mr Biden and four per cent back the expected Republican nominee. Meanwhile, 91 per cent of Republicans support Mr Trump, with seven per cent backing Mr Biden.

“When the hypothetical matchup is expanded to include independent and Green Party candidates, Biden receives 38 per cent support, Trump receives 37 per cent support, independent candidate Robert F. Kennedy, Jr. receives 15 per cent support, independent candidate Cornel West receives 3 per cent support, and Green Party candidate Jill Stein receives 3 per cent support,” Quinnipiac notes in a summary of its findings.

Sixty-seven per cent of voters think Mr Biden, 81, is too old to serve another term while 31 per cent think he’s not too elderly to go on.

In September last year, Quinnipiac found that 68 per cent thought Mr Biden was too old to continue as president while 28 per cent thought he was not.

Meanwhile, a majority of voters – 57 per cent – also thought Mr Trump, 77, is too old, while 41 per cent said he’s not too old compared to 63 to 34 per cent in September.

Asked about if the candidates have the physical fitness needed to serve a second term, 35 per cent said Mr Biden does – 60 per cent said the same for Mr Trump.

Sixty-two per cent said Mr Biden does not have the fitness required – 37 per cent said so about Mr Trump.

Mr Trump also beats Mr Biden when it comes to mental fitness – 34 per cent said yes for Mr Biden when asked if he has what it takes mentally – 48 per cent said the same about Mr Trump. Sixty-four per cent said Mr Biden doesn’t have the mental fitness needed – 51 per cent said that about the former president.

Voters were also asked about their ethics, empathy, and temperament, a category in which Mr Biden came out on top on all three questions.

Forty-nine per cent said Mr Biden was ethical – 47 per cent said no – 51 per cent said he cares about average Americans – 47 per cent said no – and 49 per cent said he has the “kind of personality and temperament it takes to serve effectively as president” – 50 per cent said he does not.

Comparatively, only 29 per cent said Mr Trump was ethical while 68 per cent said he isn’t, 42 per cent said he cares about average people, 57 per cent said he doesn’t, and only 37 per cent said he has the personality and temperament to be president while 61 per cent said he doesn’t.

Quinnipiac University Polling Analyst Tim Malloy said in a statement: “A Biden-Trump split decision on physical and emotional fitness leaves both looking vulnerable. Yes, Trump wins walking away on the age and stamina question, but voters have more confidence in Biden’s empathy toward them and his emotional stability to handle the job.”