REHOBOTH BEACH, Delaware, July 21 (Reuters) - U.S. President Joe Biden abandoned his reelection bid on Sunday under growing pressure from his fellow Democrats and endorsed Vice President Kamala Harris as the party's candidate to face Republican Donald Trump in the November election.

Biden, who at 81 is the oldest person ever to have occupied the Oval Office, said he would remain in the presidency until his term ended on Jan. 20, 2025, and would address the nation this week.

Biden, who has not been seen in public since testing positive for COVID-19 last week, was isolating at his home in Rehoboth Beach, Delaware.

"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," Biden wrote on X.

The move dramatically reshapes a White House contest that has been shaken repeatedly in the last month. Biden's disastrous June 27 debate performance drove his fellow Democrats to urge him to drop out. Then on July 13, a gunman attempted to assassinate former President Trump, 78. And last week Trump named hardline Republican U.S. Senator J.D. Vance, 39, to serve as his vice presidential running mate.

In opinion polls, Americans had expressed widespread dissatisfaction with a potential Biden-Trump rematch. Shortly after Biden's announcement, Trump told CNN: "Harris will be easier to beat than Joe Biden would have been."

Democratic National Committee Chair Jaime Harrison said the American people would soon hear from the party on next steps and the path forward for the nomination process. It was the first time in more than a half-century that an incumbent U.S. president gave up his party's nomination.

If Harris, 59, emerges as the nominee, she would become the first Black woman to lead a major-party ticket in U.S. history. A former attorney general of California and former U.S. senator, she ran unsuccessfully for president against Biden in 2020.

"My intention is to earn and win this nomination," Harris said in a statement. "I will do everything in my power to unite the Democratic Party — and unite our nation — to defeat Donald Trump."

" style="color: #000000;">Photo from AP