- Senator Michael Bennet told CNN he expected the president to lose to Donald Trump by a "landslide", but stopped short of telling him to end his candidacy.
- Questions have been raised about Mr Biden's fitness for office after a stumbling presidential debate performance against Trump late last month.
Image: WHITE HOUSE
The first Democratic senator has publicly questioned President Joe Biden's election chances, after seven congressman broke cover and urged the 81-year-old to step aside.
Senator Michael Bennet told CNN he expected the president to lose to Donald Trump by a "landslide", but stopped short of telling him to end his candidacy.
Questions have been raised about Mr Biden's fitness for office after a stumbling presidential debate performance against Trump late last month.
The president insists he can beat his adversary, and continues to hold support among key allies after congressional Democrats met to debate his leadership on Tuesday.
The US president is in an even brighter spotlight this week as he hosts a meeting of world leaders for a Nato military summit, with the issue of Ukraine aid on the table.
He is expected to face intense global scrutiny when he delivers a press conference at the meeting in Washington on Thursday afternoon.
Speaking briefly to reporters after Tuesday's congressional discussions, Senate Majority Leader Chuck Schumer repeated the message: "I'm with Joe."
Others have emphasised the importance of party solidarity. Speaking to BBC Radio 4's World Tonight programme, Hank Johnson, a congressman from Georgia, said it was time to "break the circular firing squad" around Mr Biden.
Mr Johnson is a member of the influential Congressional Black Caucus. While admitting Mr Biden had a "horrible" debate with Trump, he said the overwhelming majority of voters of colour wanted him as their candidate.
The caucus - a group of about 60 lawmakers - reportedly backed the president during a call on Monday.
On Tuesday, Mr Biden gave brief but vociferous remarks to open the Nato summit, declaring the alliance to be "more powerful than ever". Observers said he struck a markedly different tone to his fumbling performance in the 27 June debate.
But elsewhere, the dissent continues to trickle through.
After congressional Democrats emerged from their private talks, Some chose to speak to reporters, touting Mr Biden's record in the White House or speaking of Democratic unity.
But the same day, a seventh House Democrat - Mikie Sherrill of New Jersey - publicly called on Mr Biden not to run for re-election, saying the stakes were "too high".
She joined other dissenting voices in Congress that include Adam Smith, who told the BBC earlier this week that the party needed to install a "stronger messenger" as its candidate.
In his comments to CNN, Mr Bennet said the question of whether to carry on in the race was "something for the president to consider". But he said Trump was "on track, I think, to win this election and maybe win it by a landslide".
The Colorado Democrat framed the issue as "a moral question about the future of our country".
Two other senators are reported by CNN to believe that Mr Biden cannot win the White House again, although neither has publicly broken ranks.
The same day, George Stephanopoulos, a former Democratic operative turned ABC News anchor who interviewed Mr Biden last week, said he does not think the president "can serve four more years".
The audio was captured by TMZ as Mr Stephanopoulos made the comment to a passerby.
Although Mr Stephanopoulos did not rescind the comment, he later acknowledged he should not have made it.
Mr Biden reportedly also faces doubts from diplomats visiting Washington for the Nato meeting, one of whom anonymously told Reuters they could not see him staying in post for another four years.
As the debate lingers - and the White House faces questions about its approach to disclosing information about Mr Biden's health - speculation continues to grow over who would replace Mr Biden if he chose to step down.
Vice-President Kamala Harris is earning some high-profile backers, despite her ongoing vocal support for the president. During a rally in Las Vegas on Tuesday, Ms Harris insisted her boss was a "fighter".
Mr Biden himself has dared his doubters to either challenge him or unite behind his candidacy, although he has admitted he "screwed up" in the Trump debate.
His performance in the head-to-head has been ridiculed by his rival, who has suggested Ms Harris would be a "better" competitor for the White House.
But in his latest comments on the matter, Trump said he expected his adversary to stay in the race: "He’s got an ego, and he doesn’t want to quit."