Former US president Barack Obama endorsed his fellow Democrat Kamala Harris' bid for the White House on Friday, delivering a major boost to her campaign to defeat Donald Trump in November's presidential election.

The country's first woman vice president is seeking to make history again in November after President Joe Biden abruptly announced he would not seek re-election after weeks of mounting pressure on him to quit.

Barack and Michelle Obama's backing will add to the growing momentum behind Harris' campaign, with polls showing she has already narrowed the gap that existed between Trump and Biden.

"Earlier this week, Michelle and I called our friend Kamala Harris. We told her we think she'll make a fantastic President of the United States, and that she has our full support," Obama said on social media platform X.

"At this critical moment for our country, we're going to do everything we can to make sure she wins in November. We hope you'll join us."

The influential former leader and first black US president was one of the last Democratic heavy hitters to offer his endorsement, with Harris having already received the backing of Biden on Sunday.

Harris, 59, jumped into the election after weeks of turmoil over 81-year-old Biden, who bowed out after a dismal debate performance against Trump accelerated concerns over his mental capacity and persistently low polling numbers.

She has since launched a blistering attack on Trump and his "extremist" Republicans.

On Thursday, Harris addressed the American Federation of Teachers – the first union to endorse her bid – warning the country was witnessing a "full-on attack" by Trump's Republicans on "hard-won, hard-fought freedoms."

Democratic Party delegates have rapidly rallied en masse behind Harris, and the momentum behind her campaign has appeared to catch Trump off guard.

The bombastic Republican has refused to schedule a debate with Harris, saying Thursday night it would be "inappropriate" until she was officially named the Democratic nominee.

Harris, a former top prosecutor for California, chided her opponent on X, saying: "What happened to 'any time, any place?'"

She had previously said of a potential September 10 face-off: "I'm ready. So let's go." (AFP)