US Vice President and Democratic Presidential candidate Kamala Harris boards Air Force Two as she departs from Ellington Airport in Houston, Texas, on July 25, 2024. Harris is returning to Washington, DC after delivering remarks at a teachers' union event.  Brendan Smialowski / POOL / AFP)

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 beat Donald Trump in November's presidential election.

"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 was one of the last Democratic heavy hitters to offer his endorsement, with Harris having already received the backing of President Joe Biden on Sunday to take his place on the ballot.

Obama's backing will add to the growing momentum behind Harris' campaign, which has enjoyed a groundswell of support since she announced her 11th-hour candidacy.

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.

The country's first woman vice president -- who is seeking to make history again in November -- launched a blistering attack on Trump and his "extremist" Republicans as she addressed teachers Thursday.

The momentum appeared to catch Trump off guard, with the bombastic Republican refusing to schedule a debate with Harris, saying Thursday night it would be "inappropriate" until she was officially named the Democratic nominee.

"Democrats very well could still change their minds," Trump campaign spokesman Steven Cheung said in a statement.

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."

- Union endorsement -

The first union to endorse Harris -- the American Federation of Teachers -- applauded at their convention in Houston as Harris warned that the country was witnessing a "full-on attack" by Trump's Republicans on "hard-won, hard-fought freedoms."

"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.

The speech came with Harris facing increasingly extreme rhetoric from Trump, who on Wednesday called her a "radical left lunatic" and claimed -- entirely falsely -- that she was in favor 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.