United States President Joe Biden dropped out of the race for re-election on Sunday, amid mounting pressure from within the Democratic Party for him to end his campaign.

In a statement, Biden said that he had decided not to accept the party’s nomination for the presidential election scheduled for November. He said he would focus on his duties as president for the remainder of his term, which will conclude in January.

Biden endorsed Vice President Kamala Harris as the Democratic Party’s presidential nominee. Harris was also his running mate in this year’s presidential campaign.

The 81-year-old’s withdrawal follows a series of comments by senior Democratic Party leaders urging him to quit the presidential race, according to reports.

The calls grew after several party leaders and legislators were unhappy with Biden’s performance in the June 27 election debate with former President Donald Trump, who is the Republican Party’s presidential candidate this year.

Biden is the first sitting president eligible for re-election to drop out of the contest since President Lyndon Johnson did so in 1968.

“It has been the greatest honour of my life to serve as your president,” Biden wrote in a note shared on social media. “And while it has been my intention to seek re-election, 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.”

“My very first decision as the party nominee in 2020 was to pick Kamala Harris as my vice president,” Biden added. “And it’s been the best decision I’ve made. Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year.”

Reacting to Biden’s decision, Harris said that she was “honoured to have the president’s endorsement” and that she intended “to earn and win this nomination”.

It was unclear if a significant number of senior Democrats had accepted Harris as the party’s nominee or if they would look for other possible candidates.

Harris is required to be formally nominated at the party convention scheduled in August.

If she secures the nomination, Harris will be the first African American and Asian American woman to run for the office of United States president.

Biden has, several times, failed to complete his sentences or made gaffes while speaking to the press and the public, leading to questions about whether he is fit enough to stay in office for another term.

Trump, who was the president between 2017 and 2021, accepted the Republican Party’s nomination on July 19.

Biden had defeated Trump in the 2020 election.

On July 18, Biden tested positive for Covid-19, which forced him to pause his campaign and go into isolation.