Robert F Kennedy Jr sticks his hand up

US President Joe Biden has dropped his faltering reelection bid amid intensifying opposition within his own Democratic Party, and endorsed Vice President Kamala Harris to replace him as the party's candidate against Republican Donald Trump.

Biden, 81, in a post on X, said he will remain in his role as president and commander-in-chief until his term ends in January 2025 and will address the nation this week.

He has not been seen in public since testing positive for Covid-19 last week and is isolating at his home in Rehoboth Beach, Delaware.

"While it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as President for the remainder of my term," Biden wrote.

Independent candidate Robert F Jennedy Jr has this morning put his name in the ring for the Democratic nomination for president.

"I commend President Biden for stepping down. His infirmities were evident to any unbiased observer from the beginning," Kennedy wrote on X.

"It was this progressive deterioration - and his abandonment of Democratic Party principles - that prompted me to enter the race and ensure American voters had a viable, vigorous alternative to Donald Trump."

In a media conference streamed live from his home, Kennedy said a "cabal" was choosing the party's next candidate. He said he would be receptive if "party elders" asked him to challenge Harris for the nomination, US media reported.

He criticised both Harris ("monumentally unpopular") and Trump ("the swamp") in his half-hour long comments.

Democratic National Committee Chair Jaime Harrison said the American people will hear from the party on next steps and the path forward for the nomination process soon. It was the first time in more than a half-century that an incumbent US president gave up his party's nomination.

US Vice President Kamala Harris, shown in this file photo, says she intends to win the Democratic...
US Vice President Kamala Harris, shown in this file photo, says she intends to win the Democratic Party's nomination as its presidential candidate. Photo: Reuters
If officially nominated, Harris, 59, would become the first Black woman to run to lead a major-party ticket in US history.

A former attorney general of California and former US senator, she ran unsuccessfully for president against Biden in 2020.

"My intention is to earn and win this nomination," Harris said in a statement. "I will do everything in my power to unite the Democratic Party — and unite our nation — to defeat Donald Trump."

Biden's campaign had been on the ropes since a disastrous June 27 debate against former President Trump, 78, in which the incumbent at times struggled to finish his thoughts. Trump accepted his party's nomination at a convention last week.

Opposition from within Biden's party gained steam over the past week with 36 congressional Democrats - more than one in eight - publicly calling on him to end his campaign.

Lawmakers said they feared he could cost them not only the White House but also the chance to control either chamber of Congress next year, which would leave Democrats with no meaningful grasp on power in Washington.

That stood in sharp contrast to what played out in the Republican Party last week, when convention delegates united around Trump and his running mate US Senator J.D. Vance, 39.

Other prominent Democrats seen as potential vice presidential material, including Governors Gavin Newsom of California, Gretchen Whitmer of Michigan and Andy Beshear of Kentucky in statements praised Biden and vowed to fight for a Democratic win in November, without specifically mentioning their ambitions or endorsing Harris as Biden's successor.

Trump told CNN on Sunday that he believed Harris would be easier to defeat.

Biden had a last-minute change of heart, said a source familiar with the matter. The president told allies that as of Saturday night he planned to stay in the race before changing his mind on Sunday afternoon.

"Last night the message was proceed with everything, full speed ahead," the source told Reuters, speaking on condition of anonymity. "At around 1:45 p.m. today: the president told his senior team that he had changed his mind."

Biden announced his decision on social media within minutes.

It was unclear whether other senior Democrats would challenge Harris for the party's nomination - she was widely seen as the pick for many party officials - or whether the party itself would choose to open the field for nominations.

Public opinion polling shows that Harris performs no better statistically than Biden against Trump.

In a hypothetical head-to-head matchup, Harris and Trump were tied with 44% support each in a July 15-16 Reuters/Ipsos poll conducted immediately after the July 13 assassination attempt on Trump. Trump led Biden 43% to 41% in that same poll, though the 2 percentage point difference was not meaningful considering the poll's 3-point margin of error.

Republicans questions Biden's capacity to stay in power

Congressional Republicans argued that Biden should resign the office immediately, which would turn the White House over to Harris and put House of Representatives Speaker Mike Johnson, a Republican, next in line in succession.

"If he's incapable of running for president, how is he capable of governing right now? I mean, there is five months left in this administration. It's a real concern, and it's a danger to the country," Johnson told CNN on Sunday before Biden's announcement.

Johnson in a separate interview on ABC signaled that Republicans would likely try to mount legal challenges to Democrats' move to replace Biden on the ballot.

Biden's announcement follows a wave of public and private pressure from Democratic lawmakers and party officials to quit the race after his shockingly poor debate.

His troubles took the public spotlight away from Trump's performance, in which he made a string of false statements, and trained it instead on questions surrounding Biden's fitness for another four-year term.

His gaffes at a NATO summit - invoking Russian President Vladimir Putin's name when he meant Ukrainian President Volodymyr Zelenskiy and calling Harris "Vice President Trump" -further stoked anxieties.

First since LBJ

Biden's historic move - the first sitting president to give up his party's nomination for reelection since President Lyndon B. Johnson during the Vietnam War in March 1968 - leaves his replacement with less than four months to wage a campaign.

If Harris emerges as the nominee, the move would represent an unprecedented gamble by the Democratic Party: its first Black and Asian American woman to run for the White House in a country that has elected one Black president and never a woman president in more than two centuries.

A growing number of Democratic US lawmakers backed Harris in the hours after Biden's announcement as the vice president vowed to earn their support, while Republicans were quick to tie her to Biden's agenda and blame her for the crisis at the US southern border and inflation.

"The work is not done; in fact it is just beginning. I proudly and enthusiastically support Vice President Harris, whom I believe is the very best person in this moment to unify the Democratic Party and lead us forward to victory," Democratic US Senator Tina Smith wrote on X.

Biden was the oldest US president ever elected when he beat Trump in 2020. During that campaign, Biden described himself as a bridge to the next generation of Democratic leaders. Some interpreted that to mean he would serve one term, a transitional figure who beat Trump and brought his party back to power.

But he set his sights on a second term in the belief that he was the only Democrat who could beat Trump again amid questions about Harris's experience and popularity. In recent times, though, his advanced age began to show through more. His gait became stilted and his childhood stutter occasionally returned.

His team had hoped a strong performance at the June 27 debate would ease concerns over his age. It did the opposite: a Reuters/Ipsos poll after the debate showed that about 40% of Democrats thought he should quit the race.

Donors began to revolt and supporters of Harris began to coalesce around her. Top Democrats, including former House of Representatives Speaker Nancy Pelosi, a longtime ally, told Biden he cannot win the election.

Biden's departure sets up a stark new contrast, between the Democrats' presumptive new nominee, Harris, a former prosecutor, and Trump who is two decades her senior and faces two outstanding criminal prosecutions related to his attempts to overturn the 2020 election result. He is due to be sentenced in New York in September on a conviction for trying to cover up a hush-money payment to a porn star.

Biden struggled before debate

Earlier this year, facing little opposition, Biden easily won the Democratic primary race to pick its presidential candidate, despite voter concerns about his age and health.

His staunch support for Israel's military campaign in Gaza eroded support among some in his own party, particularly young, progressive Democrats and voters of color, who make up an essential part of the Democratic base.

Many Black voters say Biden has not done enough for them, and enthusiasm among Democrats overall for a second Biden term had been low. Even before the debate with Trump, Biden was trailing the Republican in some national polls and in the battleground states he would have needed to win to prevail on Nov. 5.

Harris was tasked with reaching out to those voters in recent months.

During the primary race, Biden accumulated more than 3,600 delegates to the Democratic National Convention to be held in Chicago in August. That was almost double the 1,976 needed to win the party's nomination.

Unless the Democratic Party changes the rules, delegates pledged to Biden would enter the convention “uncommitted," leaving them to vote on his successor.

Democrats also have a system of “superdelegates," unpledged senior party officials and elected leaders whose support is limited on the first ballot but who could play a decisive role in subsequent rounds.

Biden beat Trump in 2020 by winning in the key battleground states, including tight races in Pennsylvania and Georgia. At a national level, he bested Trump by more than 7 million votes, capturing 51.3% of the popular vote to Trump's 46.8%.

Additional reporting RNZ