WASHINGTON: Barack and Michelle Obama pledged support for Kamala Harris on Friday, filling the last major gap in her bid to unite Democrats around her dramatic, late-hour challenge against Donald Trump this November.

The boost for Harris came amid new turmoil for 78-year-old Trump, who cast into doubt whether he will debate the vice president.

Trump is scrambling to reorient an election he thought would be against an 81-year-old incumbent president beset by health worries, but which instead now features an energized replacement, two decades younger.

Harris, by contrast, has enjoyed a fast start since entering the race after President Joe Biden abruptly withdrew last Sunday and endorsed her.

The Obamas' public declaration of support delivers a new 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 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, and that she has our full support,“ Barack Obama announced on X.

“At this critical moment for our country, we’re going to do everything we can to make sure she wins in November.”

Seeking to become the first female president in US history, Harris faces the difficult task of 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, Harris, 59, is already making inroads. Polls that had shown Biden steadily slipping against Trump now show Harris in a race too close to call.

- No televised debate? -

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

Trump, who narrowly survived an assassination attempt at a rally earlier this month, was brimming with confidence at his party's convention, where his formal nomination alongside vice presidential pick J.D. Vance was more like a coronation.

But Biden's exit three days later rattled his campaign and reversed the concerns over age, with Trump now becoming the oldest presidential nominee in history.

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

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

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

Pete Buttigieg, the Biden administration's transportation secretary and a major Harris campaign advocate, mocked Trump for being “unable to adapt.”

“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. “Tough talk is this guy’s calling card and now there’s this extraordinary show of weakness.”