Joe Biden took to the stage at his Thursday night news conference with everything on the line – his presidency, his re-election hopes, his political life.

If those were the stakes, he barely acknowledged them at the hour-long session to mark the end of a Nato summit, having earlier introduced Ukraine's President Zelensky as "President Putin" at a separate event.

The news conference was his first unscripted appearance after a disastrous debate with his rival Donald Trump, leading to calls from several Democratic politicians and donors for him to drop out of the race for president.

Mr Biden, 81, has faced continuous questions over his age and ability to serve another term, which intensified after the debate.

But at the highly anticipated news conference, he dismissed the concerns about his campaign that were posed again and again by a room full of reporters, and promised that he was fighting not for his legacy, but to finish the job he started when he took office in 2021.

“If I slow down and can’t get the job done, that’s a sign I shouldn’t be doing it,” he said. “But there’s no indication of that yet.”

Depending on perspective, it was either a sign of dogged determination or of a man in denial about how dire his situation has become.

Minutes after the news conference finished, several more Democratic members of Congress publicly called on Mr Biden to step down, joining at least a dozen other lawmakers in the president's own party who have done so.

The question for Joe Biden's campaign is whether the floodgates will now open, or if the tide will hold.

The situation will not be helped by two excruciating gaffes that will be remembered by anyone who watched.

In his very first answer, he called his own Vice-President Kamala Harris "Vice-President Trump" – a painful faceplant in front of a national television audience.

That came just an hour after another headline-grabbing mistake at a Nato event, when Mr Biden introduced Ukrainian President Volodymyr Zelensky as "President Putin", prompting loud gasps in the audience.

He corrected the first verbal misstep involving Ukraine's leader quickly. The second one he didn’t catch, even as some reporters in the room murmured in surprise and several of his top Cabinet secretaries sat stone-faced in the front row of the audience.

Those moments - the only major stumbles in an otherwise steady if not vigorous, appearance - will surely prompt nervous Democrats to wonder if there are more gaffes to come if the president presses ahead with his campaign.

But for now at least, Mr Biden seemed the happy warrior, insisting he will push on. He laughed and smiled as he was peppered with questions, and said he could keep up with Russia's Vladimir Putin and China's Xi Jinping, even if the hoarseness and cough that had been on display during his debate two weeks ago still appeared to linger.

He again insisted he didn't need cognitive tests, telling reporters that if he even saw "two doctors or seven", his critics wouldn't be satisfied.

The election campaign, he said, had barely started, and he again repeated that he was confident he could beat Donald Trump in November's election.

The Democratic delegates who will back him officially as the party's nominee at next month's convention were free to change their minds as they pleased, he said, before mock whispering: "It's not going to happen."

He said he would consider stepping aside if his staff gave him data that he couldn’t win, but that polls still show the race a dead heat.

In that regard, he is on firm ground. An Ipsos survey released earlier on Thursday, for instance, had Mr Biden only one point behind his opponent – well within the margin of error. If there’s one thing that has been clear since the start of the year, support for the two candidates has remained remarkably stable despite unprecedented drama surrounding both men.

Polling alone won’t calm the panic that has set in among many Democratic officials, however, and the storm clouds that linger around Biden’s campaign won’t be so easily dispelled.

More Democratic politicians are waiting in the wings, according to reports, poised to announce their own break with the president, having waited until the conclusion of this Nato summit to voice their concerns.

And that’s just the first round of tests for the embattled president. He has another high-profile sit-down interview, with NBC’s Lester Holt, on Monday. Donors are anxious, and earlier on Thursday several reports suggested that even figures in the president's own campaign were plotting ways to usher their candidate toward the exit.

Despite all of this, Mr Biden made clear that it will be a challenging task to pry the nomination away from him. The 81-year-old man who at times gripped the lectern with two hands and insisted he was the "best-qualified person" to run the country is not going to exit the stage quietly.