Former President Barack Obama has also endorsed Vice President Harris.
Vice President Kamala Harris wrapped up what has been a whirlwind week in the presidential race with her campaign saying Sunday it has raised more than $200 million in less than a week.
On Thursday night, Harris met with Israeli Prime Minister Benjamin Netanyahu and took the lead in addressing the public about their discussions.
Harris has secured commitments from enough delegates to become the presumptive nominee if they all honor their commitment when voting, according to ABC News reporting.
President Biden began his remarks by noting the history of the Oval Office and the men who came before him.
"I revere this office but I love my country more," he said.
"It's been the honor of my life to serve as your president but the defense of democracy, which has a stake, I think is more important than a title."
A person who has spent time with Biden and his family over the last couple of days told ABC News that "contrary to the reports I have heard there is no anger or remorse here."
"There is in only a deep sense of pride and an enormous amount of gratitude and appreciation for the overwhelming recognition of him as a true patriot and historic defender of democracy," the source said.
"I believe that it will be remembered as one of the most selfless acts done in the interests of the greater good in the history of American politics," they added.
-ABC News' Eric Avram
Biden's bombshell announcement was delivered in a letter to Americans posted on social media on Sunday as he recovered from COVID-19 at his residence in Rehoboth, Delaware.
The post on X came just one minute after he told staff of his decision, sources told ABC News.
"It has been the greatest honor of my life to serve as your President," Biden wrote. "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."
Tonight's speech will be his first time discussing his decision at length and in front of the cameras to the nation.
About 500 White House staffers will be gathered together tonight to watch President Biden's remarks from the White House State Floor, which includes the East Room and State Dining Room.
Just before 7 p.m., there was a procession of staffers making their way over from the Eisenhower Executive Office Building, where most staffers work, to the White House North Portico entrance.
-ABC News' Karen Travers
