United States President Joe Biden has promised to remain in the presidential race and beat his opponent despite growing calls from members of his Democratic Party asking him to withdraw.

The 81-year-old, who is isolating at his beach house in Delaware since his COVID-19 diagnosis, took aim at his rival Donald Trump – who just wrapped up the Republican National Convention – in a series of posts online on Friday, and said he would return to campaigning next week.

Trump’s “dark vision for the future is not who we are as Americans. Together, as a party and as a country, we can and will defeat him at the ballot box,” Biden said. “The stakes are high, and the choice is clear. Together, we will win.”

But 12 more Democratic politicians, including two senators and a group of Congress members, called on the president to step aside.

Ohio Senator Sherrod Brown, a Democrat who is in a tough race for re-election, said in a statement late on Friday that he agrees with “the many Ohioans” who have reached out to him asking for Biden to end his campaign.

New Mexico Senator Martin Heinrich, who is also up for re-election, became the third Senate Democrat to call for Biden’s exit.

“By passing the torch, he would secure his legacy as one of our nation’s greatest leaders and allow us to unite behind a candidate who can best defeat Donald Trump and safeguard the future of our democracy,” he said.

Top Democratic figures, including Chuck Schumer, Hakeem Jeffries and Nancy Pelosi, have also privately pressured the president to quit, according to media reports.

“He’s not going anywhere,” Biden campaign chair Jen O’Malley Dillon said on MSNBC’s Morning Joe show, and called him the “best person to take on Donald Trump”.

She indicated that the growing unease among top Democrats about Biden’s candidacy did not reflect the broader sentiment across the party.

And in a call with donors on Friday, Vice President Kamala Harris said: “We are going to win this election,” US media reported. “We know which candidate in this election puts the American people first: our president, Joe Biden.”

A group of Biden’s Democratic detractors are trying to reach him directly through a television advertisement.