Obama's endorsement ends all doubt whether he will support Harris for the party's nomination.

"Earlier this week, Michelle and I called our friend @KamalaHarris. We told her we think she’ll make a fantastic President of the United States, and that she has our full support. 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 posted on X.

Kamala Harris emerged as a contender to become Democrat nominee after Joe Biden dropped out of the race on Sunday.

Biden was under pressure to quit the presidential race following his struggling performance in the debate with many within his party doubting his candidacy and re-election attempt.

Biden made the announcement just four months before the USA votes to elect a new President.

In his statement posted on X, Biden said: "It has been the greatest honor of my life to serve as your President. And while it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as President for the remainder of my term."