Senator Bernie Sanders, a prominent figure of the US political left, on Saturday threw his support behind Joe Biden’s White House campaign, dismissing calls for the president to withdraw from the race over health concerns.

“Enough! Mr Biden may not be the ideal candidate, but he will be the candidate and should be the candidate,” independent senator Sanders wrote in a New York Times column, adding that “it’s time for Democrats to stop the bickering and nit-picking.”

Sanders’s column comes amid steadily growing calls from Democratic officials and donors for the 81-year-old Biden to step aside following a disastrous performance in his June 27 debate with former president Donald Trump. So far, some 20 members of Congress have called for him to leave the race.

Most polls taken since the debate show Trump with a national lead over Biden, though within the margin of error. They also show some key battleground states, including Georgia, Arizona and Nevada, shifting in the Republican’s favor.

And a majority of Americans — including Democrats — believe Biden is too old to serve again, according to a recent poll.

The president did not help his own cause when, at a summit in Washington earlier this week, he mistakenly introduced Ukrainian President Volodymyr Zelensky as his foe Vladimir Putin, before quickly correcting himself.

Biden, however, insisted during an appearance Friday that he is, and will remain, his party’s flagbearer.

“I am running and we’re going to win,” he told an enthusiastic audience in Detroit, amid chants of “Don’t you quit.”

His running mate and Vice President Kamala Harris doubled down her support for Biden, calling him a “fighter” at a campaign event on Saturday in the swing state of Pennsylvania, where Trump is also holding a rally.

“So, we will continue to fight. We will continue to organize. And in November, we will win,” said Harris, who some pundits have speculated could lead the Democratic ticket if Biden stepped aside.

Seeking to allay concerns, Biden spoke with members of the Democratic Party’s left-leaning Congressional Progressive Caucus in a virtual meeting Saturday.

He will also sit for interviews next week with NBC News and Black Entertainment Television (BET).

‘Circular firing squad’

Sanders, who at 82 is himself seeking reelection to the Senate in November, did acknowledge the signs of the president’s aging.

But given Biden’s achievements — and the record of Trump, “who has 34 felony convictions and… has told thousands of documented lies” — the senator brushed Biden’s shortcomings aside.

“Yes, I know,” he wrote. “Mr Biden is old, is prone to gaffes, walks stiffly and had a disastrous debate with Mr Trump. But this I also know: A presidential election is not an entertainment contest. It does not begin or end with a 90-minute debate.”

He blamed the eruption of anti-Biden rhetoric partly on “the corporate media (which) has obsessively focused on the June presidential debate and the cognitive capabilities of a man who has, perhaps, the most difficult and stressful job in the world.”

Sanders also blamed those Democrats who he said had “joined that circular firing squad.”

He challenged the president to do more — to “propose and fight for a bold agenda” that would help lift working families “who have been left behind for far too long.”

But Biden, he concluded, was “a good and decent Democratic president with a record of real accomplishment,” including helping rebuild the US economy and shoring up the country’s crumbling infrastructure.

“For the sake of our kids and future generations, he must win.”