Biden's faltering performance at a June 27 debate crystalized longstanding concerns about the 81-year-old president's physical and mental stamina, causing a substantial loss of confidence among voters heading into the general election campaign.

"It has been the greatest honour of my life to serve as your President," Biden said in a letter posted on X.

"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.” 

Biden, who was previously the first Catholic vice president, and later became the second Catholic president in U.S. history, has previously been at odds with the U.S. bishops over his administration's abortion policy but has won some bishops' praise on refugee and climate-related policies, as well as mixed responses to his policies on immigration.

Biden argued the nation has made "great progress" during his time in office, arguing "America has the strongest economy in the world." He cited some of his accomplishments, including "lowering prescription drug costs for seniors, the first gun safety law in 30 years," and appointing "the first African-American woman to the Supreme Court."

Biden said he will address the nation in the coming week to share "more detail about my decision."

"And let me express my heartfelt appreciation to the American people for the faith and trust you have placed in me," he said. "I believe today what I always have: that there is nothing America can't do — when we do it together. We just have to remember we are the United States of America."

Biden's letter did not shed light on next steps for his party. But in a subsequent post on X, Biden endorsed his Vice President Kamala Harris to be the Democratic Party's presidential nominee in November.

Harris in a statement praised Biden for "his extraordinary leadership" and said it was "a profound honour to serve as his Vice President." Harris then said she was "honoured to have the President's endorsement and my intention is to earn and win this nomination."

In a statement posted to his social media platform Truth Social, former President Donald Trump, the Republican presidential nominee, said Biden "was not fit to run for President, and is certainly not fit to serve — And never was!"

Trump criticized Biden's immigration policy, and argued, "We will remedy the damage he has done very quickly."

While it is not entirely without precedent in American history for an incumbent president to end his reelection campaign, such decisions are rare, and none have occurred because of pressure from within the incumbent's party about his mental and physical capabilities to do the job.

The announcement signals the end of a political career in Washington spanning decades. Biden previously represented Delaware for 36 years in the U.S. Senate, first elected at age 29, before his election as vice president in 2008, a role he held for two terms during former President Barack Obama's administration. In 2020, he was elected as the 46th president of the United States.