US President Joe Biden said on Wednesday night in a nationally televised address that he decided not to seek a second term because he wanted to "pass the torch to a new generation", which he said is "the best way" to unite the nation.

In the 13-minute speech, Biden made references to the danger he said the United States would face if Donald Trump wins the Nov 5 election, without naming the former president and Republican nominee.

He said he was dropping out of the race against Trump to help heal the wounds in the Democratic Party and unite Democrats in the goal of winning.

"Nothing can come in the way of saving our democracy. That includes personal ambition," he said during a prime-time address in the Oval Office.

"You know, there is a time and a place for long years of experience in public life. But there's also a time and a place for new voices, fresh voices, yes, younger voices and that time and place is now," he said.

It was Biden's first explanation about his abrupt departure from the 2024 presidential race and capped more than 50 years in politics.

Biden said that during the last six months of his time in office, he will be focused on his job as president, from working to grow the economy to "defending our personal freedoms".

The president touched on a lot of the themes of his campaign.

The political aspects of the message prompted Chris LaCivita, a senior adviser to Trump, to post on X: "Every single network that televised this campaign speech will be getting follow-up letters from our attorneys regarding equal time."

Biden said he would call for Supreme Court reform in his final months in office.

The Supreme Court and its 6-3 conservative majority has in recent years made a series of landmark decisions, overturning Roe versus Wade on abortion rights, limiting affirmative action and granting wide immunity to presidents for official acts taken while in office.

Praise for Harris

Biden also praised Vice-President Kamala Harris, 59, the likely Democratic nominee, as a strong leader who would make an effective president, saying that she is experienced, tough and capable.

The speech comes after he dropped his bid early Sunday afternoon in a letter addressed to "my fellow Americans", the first sitting president since Lyndon B. Johnson in 1968 not to seek a second term in the White House.

Then he announced the decision in a post on X and he followed that moments later with an endorsement of Harris for the nomination.

Harris has consolidated support from within the Democratic Party and is expected to win a virtual roll call of Democratic delegates — perhaps without opposition — as early as Aug 1 and no later than Aug 7.

Biden returned to the White House on Tuesday after testing negative for COVID-19. He began drafting his anticipated address while isolating with the virus in Rehoboth Beach, Delaware, senior officials told CNN.

A Reuters/Ipsos poll completed on Tuesday showed Harris with a 2 percentage-point lead over Trump, 44 percent to 42 percent. A CNN poll conducted by SSRS showed Trump leading Harris, 49 percent to 46 percent. Both findings were within the polls' margins of error.

The Harris campaign on Wednesday said it had raised $126 million since Sunday, with 64 percent of donors making their first contribution of the 2024 campaign.

Agencies contributed to this story.