A new poll suggests that some Democratic voters are increasingly concerned about whether Joe Biden is mentally fit to serve as president after a shaky debate performance on Thursday.

A CBS News/YouGov poll released on Sunday indicated that 72 percent of registered voters believe the president does not have the mental and cognitive health to serve as president - a sharp increase from the 65 percent who said the same in an earlier poll.

Forty-nine percent of voters said the same of former President Donald Trump. Particularly alarming for the Biden campaign, 45% of registered Democrats who responded to the poll said they believe the president should step aside for another candidate.

Concerns about the candidates' age - Mr Biden's is 81 and Trump is 78 - existed before Thursday's debate. Mr Biden's hoarse voice and muddled answers, however, renewed concerns among some Democrats about his candidacy and left some calling for Mr Biden to step aside. The president's family encouraged him to stay in the race and keep fighting during a long-planned visit to Camp David on Sunday, a source with knowledge of the conversations confirmed to CBS News, BBC's US partner.

Still, Maryland Congressman Jamie Raskin, a Democrat and White House ally, called it a "difficult situation" in an interview with MSNBC on Sunday. He said that there are "very honest and serious and rigorous conversations taking place at every level of our party", though he emphasised that the ultimate decision remained Mr Biden's.

“Regardless of what President Biden decides, our party is going to be unified and our party also needs him at the very centre of our deliberations in our campaign," he added.In the days since the debate, first lady Jill Biden, Vice President Kamala Harris and former President Barack Obama have publicly acknowledged it was not Mr Biden's best performance.

But they and many other Democratic allies rejected calls that Mr Biden should step aside.House Minority Leader Hakeem Jeffries told MSNBC on Sunday that the president's debate performance "was a setback. But, of course, I believe a setback is nothing more than a setup for a comeback". This poll from CBS News and polling organisation YouGov contrasts with a memo shared by Biden campaign chairwoman Jen O’Malley on Saturday. It claims that internal polling shows that it is the "beltway class" counting Mr Biden out, not Americans across the country.