WASHINGTON: 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."

Teachers' union members applauded Harris as she addressed them in Texas.

"While you teach students about democracy and representative government, extremists attack the sacred freedom to vote. While you try to create safe and welcoming places where our children can learn, extremists attack our freedom to live safe from gun violence," she said.

"They have the nerve to tell teachers to strap on a gun in the classroom while they refuse to pass common sense gun safety laws."

Harris tied the event to a key campaign message about refusing to go back to Trump's America, praising her audience as "visionaries" who look to the future.

And she contrasted Democratic efforts to cancel student debt and her vision of investment in public schools and universities with Trump's vow to dismantle the Education Department and cut spending in half.

The speech came with Harris facing increasingly extreme rhetoric from Trump, who on Wednesday called her a "radical left lunatic" and claimed - falsely - that she was in favour of the "execution" of newborn babies.

Trump, who at 78 is the oldest presidential nominee in US history, has promised he will "not give one penny" of federal funds to schools with vaccine mandates. Every public school in America has such mandates.

Trump and Harris were statistically tied in a new New York Times/Siena College poll that showed her narrowing the gap after the survey found Biden behind by six points in early July.

One of the most urgent tasks facing her in the much shorter term, however, is to forge her own political identity before she can be defined by Trump as inseparable from the unpopular Biden.

And she has begun quickly spending some of the US$100 million-plus that she has raised in recent days to tell her personal story and counter Republican characterizations of her as an out-of-touch liberal.

 -AFP