Former US President Barack Obama endorsed fellow Democrat Kamala Harris' bid for the White House on Friday, giving a significant boost to her campaign against Donald Trump in the upcoming November 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 announced on the 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."

Obama was one of the last prominent Democrats to endorse Harris, who had already received backing from President Joe Biden on Sunday. This endorsement will further strengthen Harris' campaign, which has seen a surge of support since her last-minute entry into the race.

Harris, 59, entered the election after weeks of controversy surrounding 81-year-old Biden. His poor debate performance against Trump heightened concerns over his mental fitness and low polling numbers, leading him to withdraw from the race.

The nation's first female vice president, Harris is aiming to make history again in November. She delivered a fierce critique of Trump and his "extremist" Republicans during a speech to teachers on Thursday.

Trump appeared caught off guard by Harris' momentum, refusing to schedule a debate with her, stating it would be "inappropriate" until she was officially named the Democratic nominee. "Democrats very well could still change their minds," said Trump campaign spokesman Steven Cheung.

Harris, a former top prosecutor in California, challenged her opponent on X, questioning, "What happened to ‘any time, any place?'" She had previously expressed her readiness for a potential September 10 debate, saying, "I’m ready. So let’s go."