Tectonic moment as Joe Biden drops out of presidential race

On July 22 President Biden, 81, announced his decision on X, formerly Twitter, saying he would focus "all my energies on my duties as President for the remainder of my term".

Biden offered his "full support and endorsement" for US vice-president Kamala Harris to become the Democratic Party's nominee for the November 5 election against former president Donald Trump, 78.

"It has been the greatest honour of my life to serve as your President. And while it has been my intention to seek re-election, 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.

Vice President Harris thanked Joe Biden for his "extraordinary leadership" as president and accepted his endorsement nomination, saying her "intention is to earn and win this nomination".

"I will do everything in my power to unite the Democratic Party - and unite our nation - to defeat Donald Trump."

First Lady Jill Biden reacted to the news by reposting her husband's statement on her private X account adding an emoji showing two hearts.

Prime Minister Anthony Albanese responded in his own post to X, thanking President Biden for his leadership and ongoing service as the US president.

"The Australia-US Alliance has never been stronger with our shared commitment to democratic values, international security, economic prosperity and climate action for this and future generations."

Australia's ambassador to the United States Kevin Rudd also thanked Mr Biden, with the former prime minister saying "leadership can be a lonely place".

"We can all take pause to recognise, and respect, the difficult decision President Joe Biden has made today," Mr Rudd said.

Former President Barack Obama praised President Joe Biden's decision to abandon his reelection but stopped short of endorsing Vice President Kamala Harris as his replacement.

Obama called Biden, his former vice president, "one of America's most consequential presidents, as well as a dear friend and partner to me."

Obama said Sunday that when he picked Biden as his running mate in the 2008 campaign, "what I came to admire even more was his character - his deep empathy and hard-earned resilience; his fundamental decency and belief that everyone counts."

Former President Bill Clinton and 2016 Democratic nominee Hillary Clinton issued a joint statement endorsing Kamala Harris, saying it's time to "fight with everything we've got" to elect her.

The Clintons said Biden had "lifted America out of an unprecedented pandemic, created millions of new jobs, rebuilt a battered economy, strengthened our democracy, and restored our standing in the world".

Republican presidential candidate Donald Trump reacted to the news in a post on his social media site saying Biden "was not fit to run for President, and is certainly not fit to serve".

"We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly," he added. "MAKE AMERICA GREAT AGAIN!"

Biden's decision to endorse Harris ahead of the August Democratic convention is expected to make the process less chaotic than a time-consuming foot race between top Democrat leaders, according to reports.

That's because the vice president co-headlines the presidential ticket, and the campaign's $135 million war chest would seamlessly pass to her, providing a major advantage over other candidates.

A series of gaffes and blank moments have engulfed Biden's presidency in the last few weeks, including a disastrous TV debate and the NATO summit on July 11, where he called Ukraine's President Volodymyr Zelenskiy "President Putin" and called vice-president Kamala Harris "vice-president Trump".

The momentous decision to step down follows the political fallout from the disastrous June 27 debate where it appeared he was disengaged and unable to fact-check rival Donald Trump.

Speaking at a campaign event on July 2 President Biden admitted the TV debate against Trump did not go well.

"I didn't have my best night, but the fact is that you know, I wasn't very smart," Biden said, speaking at the campaign fundraiser without the aid of a teleprompter.

"I decided to travel around the world a couple times, going through around 100 time zones ... before ... the debate.

"Didn't listen to my staff and came back and nearly fell asleep on stage," he said. "That's no excuse but it is an explanation."

But those justifications had not appeased his Democratic colleagues on Capitol Hill.

On July 18, top Democrat Adam Schiff publicly called for him to exit the race and, according to reports, Democratic Senate Majority leader Chuck Schumer "forcefully made the case" for Biden to step aside in a one-on-one conversation.

It was also reported that Barack Obama, Democratic House of Representatives leader Hakeem Jeffries and former speaker Nancy Pelosi had held similar conversations with the president.