President Joe Biden, right, and Republican presidential candidate former President Donald Trump, left, stand during break in a presidential debate hosted by CNN, June 27, 2024, in Atlanta (AP)

US President Joe Biden's performance during a debate against challenger Donald Trump has left some Democrats questioning his viability as the party's nominee, CNN reported, citing Democratic sources.

One source mentioned that it is hard to see how Biden can become the party's nominee and another source characterized him as "incoherent", according to the report on Thursday.

One Democratic operative said, according to CNN, that it was hard for the party to argue that Biden should be their nominee, while another said, "nothing good" came out of Biden's performance in the debate.

David Axelrod, a top advisor to former President Barack Obama, told CNN, "There is a sense of shock at how he came out at the beginning of this debate. How his voice sounded. He seemed a little disoriented. ... There are going to be discussions about whether he should continue."

"Only he can decide if he’s going to continue," he maintained.

Axelrod predicted that Biden wouldn’t be inclined to leave the race, noting that "this is a guy with a lot of pride... who believes in himself."

The sources also compared Biden's performance with that of Trump as he stumbled several times on stage, often time stuttering and speaking incoherently while also making false statements, and his disoriented appearance sparked extensive criticism on social media.

"This was like a champion boxer who gets in the ring past his prime and needs his corner to throw in the towel," said one Democratic lawmaker, which meant, according to him, that Biden should exit the race.

It's "time to talk about an open convention and a new Democratic nominee," said a second Democratic lawmaker who has been an avid Biden supporter.

The fear among these Democrats is that the version of Biden that showed up to the debate cannot win in November.

"The chatter of replacement is absolutely going to explode," said one veteran Democratic strategist who has worked on presidential campaigns. “There is no coming back from this disaster.”

Meanwhile, top Biden allies dismissed the prospect of a change at the top of the ticket. California Gov. Gavin Newsom, a Democrat who many in the party see as a potential Biden alternative or a future presidential candidate, said “no” when asked if he would urge Biden to end his campaign.

"This is just bad, no matter how you spin it," said one veteran Democratic operative. "But everyone knows it’s too late to switch. But the donors will make those decisions, as they always do. Hence why we got Biden" in 2020.

On false, unchecked statements, President Biden claimed that no US troops had died abroad during his presidency. "Truth is, I'm the only president… that doesn't have any, this decade, troops dying anywhere in the world," Biden said. However, this statement overlooked the deaths of 13 US military service members during the withdrawal from Afghanistan on August 26, 2021, and three US troops killed by a drone attack in Jordan on January 28, 2024.

His stuttering and incoherent speech was also mocked by his rival, who said he could not understand what Biden was trying to say at some point, adding that he did not think the President could understand what he was saying himself, either.

Additionally, The New York Times reported that Democrats see Biden's debate performance as disastrous, citing three veteran Democratic presidential campaign officials.

One of these sources reportedly said that Biden looked scared on the debate stage. Another source called Biden's performance an "emperor has no clothes" moment, essentially criticizing the Democratic Party and the Biden administration for their inaction regarding the President's growing inability to present properly in public.

Pulitzer Prize winner investigative journalist Seymour Hersh said over a week ahead of the debate that Democratic Party leaders were reportedly contemplating plans for Biden to step aside from the Democratic presidential nomination if his performance in the debate was deemed inadequate.

Hersh's report underscores significant concern within the Democratic leadership and among major party donors regarding Biden's ability to secure a victory against former President Donald Trump in the upcoming November election.

"One extreme possibility in the case of a very bad showing Thursday night," Hersh explained, "is to obtain agreement from Biden and his family advisors for the president to come to the Democratic convention in Chicago in August and accept the accolades of a first-round delegate victory; then he would decline the nomination and throw the nominating process open to all."

This potential scenario could open the door for other prominent Democratic figures, such as California Governor Gavin Newsom and Illinois Governor J.B. Pritzker, who Hersh mentioned as potential replacements.