US President Joe Biden has expressed his intention to leave the race for November's presidential election. He says he wants to offer his "full support and endorsement" for Vice President Kamala Harris to be the Democratic Party nominee for the presidency.

Biden made the announcement on his X account on Sunday. He wrote that he believes it is "in the best interest" of his party and the country for him to "stand down" and to focus solely on fulfilling his duties as president for the remainder of his term.

Biden added that he will speak to the nation later this week in more detail about his decision.

Calls had been growing within the Democratic Party for Biden to quit his campaign after he displayed a faltering performance during a televised debate against Republican rival Donald Trump late last month.

Biden had already acquired more than 90 percent of state delegates in the contest to choose the Democratic candidate. He was set to become the party's official candidate by giving a nomination acceptance speech at the party's national convention in mid-August.

It is believed that Biden's withdrawal will allow about 4,000 state delegates to vote at their own discretion in a process to select the party's candidate.

This is the first time in 56 years that an incumbent president seeking reelection has dropped out of an election campaign. The last president to have done so was Lyndon Johnson in 1968.