Joe Biden's presidential campaign was in full damage-control mode Friday as he sought to recover from a disastrous debate showing that triggered alarm among Democrats and talk of replacing the 81-year-old on the White House ticket.

The verdict on Biden's performance against Donald Trump was damning, with party strategists admitting "consternation" within the ranks as the election looms a little over four months away.

"Democrats panic over Biden, doubting his future," ran the Washington Post headline.

Thursday evening's showdown with Trump had been touted as an opportunity for Biden to allay qualms about his advanced age, and to paint his rival as an existential threat to US democracy.

Instead, his hesitant delivery, which saw him tripping over words and losing his train of thought, underlined concern about his ability to serve another term and raised the question of whether he should even remain the party nominee.

Throughout the 90-minute face-off, Biden struggled to counter a typically bombastic performance from Trump who seemed energized and forceful in comparison as he delivered -- unchallenged -- a reel of false or misleading statements about everything from the economy to key electoral issues like immigration.

A split screen often showed Biden staring, open mouthed, as Trump held forth.

"I thought it was sad," Trump supporter Rhonda Kundrat, 51, told AFP as she waited for her candidate to speak at a rally in Virginia on Friday.

"There's no way they should have put that old man out there."

'Heartbreaking'

Thomas Friedman, a New York Times columnist who is close to the president, said the debate was "a heartbreaking moment."

"Joe Biden, a good man and a good president, has no business running for re-election," he said. "It's time for him to keep the dignity he deserves and leave the stage at the end of this term."

As both candidates returned to the campaign trail, Biden headed to the battleground state of North Carolina.

Even as the critics were quick to plunge their knives into his performance, Biden insisted he had done "well" as he stopped at a Waffle House with First Lady Jill Biden for post-debate snacks.

He attributed his raspy-voiced delivery to a "sore throat" and added that, in any case, it was "hard to debate a liar."

But as the disappointment over Biden's showing registered with Democrats, there was growing talk of how to find a new candidate before the party's convention in August.

Democrats -- including members of his own administration -- traded "frenzied" phone calls and text messages as the debate unfolded, the Times said.

So far, no senior Democratic figure has publicly called on Biden to withdraw, with most toeing a party line about sticking with the existing ticket.

"I will never turn my back on President Biden," said California Governor Gavin Newsom, who has figured prominently on lists of possible replacement candidates.

"I don't know a Democrat in my party that would do so," Newsom told reporters in the spin room after the debate.

Analysts say forcing a change in the ticket would be politically fraught, and Biden would have to decide himself to withdraw to make way for another nominee before the party convention.

A strong -- but not automatic -- candidate to take Biden's place would be his vice president, Kamala Harris, who loyally defended his performance Thursday while acknowledging he had made a "slow start."

Republican politicians sought to project calm assurance as the Democrats scrambled.

House Speaker Mike Johnson, a senior party figure, said it was clear that Biden was not "up to the job."

"Donald Trump is the only man on that stage that's qualified and capable of serving as the next president. The election cannot get here soon enough."