Allies of Kamala Harris, the vice-president, have been talking up the possibility of her replacing Mr Biden on the Democratic ticket in November, but Ms Harris has said he should not stand down.
On Monday, the US government released a letter explaining why a leading neurologist had visited the White House eight times in the last year, after it was suggested Mr Biden may have received treatment for Parkinson’s disease.
Dr Kevin Cannard, the neurologist, was treating US troops stationed in the building, the letter said.
Some Democrats remain concerned about polls showing Mr Biden is significantly less popular than his colleagues running in congressional races in swing states.
A survey conducted by AARP and released on Tuesday found Mr Biden had the support of 38 per cent of voters in Wisconsin, six percentage points behind Trump, while Tammy Baldwin, the Democratic senator, had the support of 50 per cent against her Republican rival.
Mr Biden was due to meet with Democratic mayors on Tuesday night after addressing leaders of the US’s Nato allies in Washington.
He has been criticised for taking too long to engage with elected members of his party after the concerns about his age first emerged.