After suffering a drubbing in Iowa and bizarrely conceding he will likely lose New Hampshire's Democratic primary next Tuesday, Joe Biden is under immense pressure to turn his floundering US presidential campaign around.

The popular former US vice president has been the national frontrunner for more than a year, but his enviable position is increasingly under threat, and rivals -- including a socialist-leaning senator and a small-city mayor -- smell blood in the water.

The 77-year-old moderate is being eclipsed on the campaign trail by mostly younger candidates with sharper stump speeches, more hustle, and telegenic debate performances.

An average of polling in New Hampshire, which votes Tuesday in the first-in-the-nation primary (following Iowa's troubled caucuses this past week), shows Biden plunging from first place to third, behind Bernie Sanders and Pete Buttigieg.

"To not be concerned I think would be foolish," acknowledged Biden-backer Will Johnson, a 23-year-old law student from northern Pittsburgh, said in one of several conversations voters had with AFP about the candidate.

"Hopefully a more concerted effort is made by the campaign to really push out the vote and get people to support Biden," he said, adding Biden was "the best shot we have" at defeating President Donald Trump in November.

The big crowds, however, are going to Buttigieg, 38, and Sanders, 78, the two candidates who won the most votes in Iowa and are leading in New Hampshire.

Biden took a day off the trail Thursday, five days before the primary, to huddle with top advisors about the path forward.

But instead of coming out swinging at the start of the next day's debate, Biden committed a glaring unforced error.

"I took a hit in Iowa, and I'll probably take one here," Biden said onstage, startling Johnson and other supporters who are looking for him to grab the reins, not admit defeat.

Biden, who passed a 2016 presidential run after his son Beau's death, has seen his path to this year's nomination transform into a rocky road.

He and his other son Hunter were drawn into the Trump impeachment saga, with the president pressuring Ukraine to investigate them over Hunter's involvement with a Ukrainian energy firm while his father was vice president.

No evidence of wrongdoing has emerged. But Trump has blasted the Bidens as corrupt, and some Republicans have repeatedly labeled the younger Biden's board position as a conflict of interest.

'Uphill battle'

Biden's unsteady performance has put some in the Democratic establishment on edge.

Is the situation alarming? "Yeah," former Chicago mayor Rahm Emanuel, who held a senior position in the Obama administration, told AFP.

"If certain candidates repeatedly come in third or fourth, the reason -- the logic -- for staying in the race is very hard."

After the debate, Biden's senior advisor Symone Sanders went into damage control.

"We know it might be an uphill battle but the reality is we are still in this race," she told reporters, noting how the demographics of the states that vote next are more representative of America's diversity than overwhelmingly white Iowa and New Hampshire.

On Saturday Biden intensified his attacks on Buttigieg with a withering ad belittling his experience as a small-city mayor while touting his successes as vice president.

And he used his appearance at a Manchester theater to go after Sanders' political persuasion.

If the senator wins the nomination, Biden said, every Democrat seeking election "will have to carry the label that Senator Sanders has chose for himself... 'Democratic socialist,'".

Reading his attack lines off a teleprompter, the event lacked spontaneity and grassroots appeal -- leaving some unconvinced.

"I've always loved Joe Biden, but he has to prove himself to me," said Mary Aarons, a 61-year-old publishing executive and undecided voter.

"I feel like he's running so much on his past record... more than what he's going to do in the future," added Nicole Clegg, 45, a local schoolteacher who said she too was undecided.

Senator Chris Coons, who has endorsed Biden, recalled that Bill Clinton "lost 10 out of the first 11 primaries" in 1992 before eventually winning the White House.

"This is not over," Coons said. Biden "is going to have to work harder and fight tougher and draw sharper distinctions, but he's ready and able to do that."