WASHINGTON: Barack Obama pledged support for Kamala Harris on Friday, filling the last major gap in her bid to unite Democrats around her dramatic presidential election run, while Donald Trump announced he will revisit the scene of his near-assassination.

The boost for the vice president from both Barack and Michelle Obama came as 78-year-old Trump urged supporters in a social media post to "FIGHT, FIGHT, FIGHT!"

Trump is scrambling to reorient an election he thought would be against President Joe Biden, an 81-year-old incumbent beset by concerns over his mental acuity. He now finds himself the oldest presidential nominee in history and facing an energised replacement, two decades younger.

Just last week, Trump was riding high as he accepted a hero's welcome - and the official nomination - at the Republican convention in Milwaukee.

This came a week after a gunman narrowly missed him at a rally in Butler, Pennsylvania - an extraordinary incident that Trump vowed Friday to commemorate with "a big and beautiful" new rally in the town, although he did not give a date.

But Kamala Harris is enjoying a fast and remarkably smooth start after entering the race in the wake of Biden's abrupt withdrawal last Sunday, following weeks of falling poll numbers and mounting worry over his health.

For Kamala Harris, the Obamas' public declaration of support was a welcome push.

The Democratic establishment's most revered power couple waited until all the other heavy hitters had come forward, finally making their move in a video released early Friday that shows Kamala Harris taking their call.

"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," Barack Obama announced on X.

Seeking to become the first female president in US history, Kamala Harris is tasked with rapidly assembling a campaign against an opponent who has been in near permanent campaign mode since he became president in 2016, defeating another woman - Hillary Clinton.

But while Trump has a powerful base and a head start, Kamala Harris is making inroads. Polls that had shown Biden steadily slipping against Trump now show Kamala Harris in a race too close to call.

She also got an enthusiastic nod from US Olympic icon Allyson Felix, who told AFP in Paris on Friday that a Black woman winning the White House would be "monumental."

A top California prosecutor and senator before being elected the country's first female and first Black and South Asian vice president, Kamala Harris has highlighted Trump's criminal conviction and what she said Thursday is a Republican attack on "hard-fought freedoms" in US society.

And at just 59, she is flipping the age narrative that Trump had counted on as a campaign message to bring down Biden.

The shakeup has prompted Trump to lash out at Kamala Harris in extreme language, including calling her a "radical left lunatic" and claiming - falsely - that she is in favour of the "execution" of newborn babies.

Democrats leapt on the Trump campaign announcement late Thursday that cast into doubt whether he will debate Kamala Harris.

A second Trump-Biden televised debate had been scheduled for September 10. This was expected to remain in place, with Kamala Harris replacing Biden, but Trump spokesman Steven Cheung said it was "inappropriate" to schedule when Kamala Harris was not yet formally the Democratic nominee.

Pete Buttigieg, the Biden administration's transportation secretary and a major Kamala Harris campaign advocate, mocked Trump.

"It shows that he's afraid. It shows that he knows that if the two of them are on a stage together, it's not going to end well for him," he said on MSNBC on Friday. "Tough talk is this guy's calling card and now there's this extraordinary show of weakness."

Meanwhile, Trump's vice presidential pick J.D. Vance remained entangled in bitter Democratic attacks on a resurfaced statement he made dismissing Kamala Harris and other key Democrats as "childless cat ladies" who want to make the country "miserable."

On Friday, he refused to apologise, only saying that he been "sarcastic" and that he still believes Democrats are "anti-family."

 -AFP