US President Joe Biden says dropping out of the race for the presidency and endorsing Vice President Kamala Harris as the Democratic candidate was the best way to unite the country, despite his ambition to win a second term.

Biden’s announcement on Sunday to not seek re-election followed a disastrous June debate with Trump that exacerbated questions about his ability to win, or to serve another four years if he succeeded.

In his address to the nation from the Oval Office on Wednesday night (local time), he said he had believed he earned re-election because of his record during his first term.

“But nothing, nothing can come in the way of saving our democracy. That includes personal ambition,” he said.

“I decided the best way forward is to pass the torch to a new generation. That is the best way to unite our nation.”

Harris, the first black woman and Asian-American to serve as vice president, will become the first woman elected president if she prevails on November 5. In the three days since Biden’s decision, Harris has pulled in broad support across the Democratic Party and revitalised its election campaign.

Biden praised Harris as a strong leader who would make an effective president.

“She’s experienced, she’s tough, she’s capable. She’s been an incredible partner to me and a leader for our country. Now the choice is up to you the American people.”

Biden said he intended to focus on his work as President in the six months left in his term.

“That means I will continue to lower costs for hard-working families and grow our economy. I will keep defending our personal freedoms and our civil rights – from the right to vote to the right to choose,” he said, outlining some themes that Harris is expected to build her campaign around.

Biden returned to Washington on Tuesday after isolating with Covid-19 at his home in Delaware, where he made the announcement that he was ending his campaign.

The Democratic National Committee’s rules committee agreed on Wednesday on a plan to formally nominate Harris as soon as August 1 – before the party’s August 19-22 convention in Chicago – with Harris picking a running mate by August 7.

Earlier on Wednesday, the 59-year-old Vice President called on a rally of more than 6000 black women in Indianapolis to help her revitalise the Democratic campaign.

Harris spoke at an event hosted by the Zeta Phi Beta Sorority, which was founded at Howard University, the historically black college she attended. She hopes to tap sororities’ multigenerational network of black women – who played an important role in Biden’s 2020 victory – to deliver strong voter turnout for Democrats again in November.

“I thank you. And now, in this moment, our nation needs your leadership once again,” Harris said.

Trump, coming off a triumphant week in which his party unified around his presidential bid after a failed assassination attempt two weekends ago, has had to watch as Biden’s sudden departure from the race shifted the narrative and sparked a surge of attention toward Harris at his expense.

Harris and Trump are closely competitive, public opinion polls showed this week.

A Reuters/Ipsos poll completed on Tuesday showed Harris with a marginal two-percentage-point lead over Trump, 44 per cent to 42 per cent. A CNN poll conducted by SSRS showed Trump leading Harris, 49 per cent to 46 per cent. Both findings were within the polls’ margins of error.

The Harris campaign said on Wednesday it had raised $US126 million ($191 million) since Sunday, with 64 per cent of donors making their first contribution of the 2024 campaign.