UPDATE, 4:30 p.m. ET: "My intention is to earn and win this nomination," Harris said in a statement Sunday, NBC News reported. Harris also praised Biden for his "extraordinary leadership" and added that she's honored to have his endorsement to replace him as the Democrats' presidential nominee, the news network also noted.

"I will do everything in my power to unite the Democratic Party — and unite our nation — to defeat Donald Trump and his extreme Project 2025 agenda," Harris also said, according to NBC News.

UPDATE, 4:17 p.m. ET: Former President Barack Obama made no mention of Harris or offered any endorsement for a Democrat presidential nominee in a statement Sunday that praised Biden, who dropped out of the race just a few hours prior.

Obama wrote that "we will be navigating uncharted waters in the days ahead. But I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges."

He added that Biden — who served as Obama's vice president — "has never backed down from a fight. For him to look at the political landscape and decide that he should pass the torch to a new nominee is surely one of the toughest in his life. But I know he wouldn’t make this decision unless he believed it was right for America."

UPDATE, 3:46 p.m. ET: Former President Bill Clinton and former Secretary of State Hillary Clinton endorsed Harris for president, saying in a statement they would "fight with everything we’ve got" to get her elected, ABC News reported.

The Democrat couple also said, "We are honored to join the President in endorsing Vice President Harris and will do whatever we can to support her," ABC News added.

UPDATE, 3:36 p.m. ET: Former President Donald Trump — who accepted the Republican nomination for president at his party's convention last week, just days after surviving an assassination attempt in Pennsylvania — told NBC News in a phone interview Sunday that Biden "should never have been there in the first place."

Trump called Biden "the worst president in the history of the United States by far," the news network noted, and also said Biden "should have stayed in his basement."

Trump’s campaign said Biden “quit the race in complete disgrace," NBC News also reported.

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In a statement posted to X, formerly known as Twitter, President Joe Biden announced that he has decided not to complete his campaign for re-election to the presidency in 2024.

Biden, who has faced a mounting crescendo of pressure from party leaders after a series of post-debate television appearances failed to convince Democratic Party leaders of his ability to defeat former president Donald Trump in the general election, said in his statement, "It has been the greatest honor in my life to serve as your president. And while it has been my intention to seek reelection, I believe it is in the best interests of my party and the country for me to stand down and focus solely on fulfilling my duties as president for the remainder of my term.

While Biden's statement thanked Vice President Kamala Harris for her service, it stopped short of endorsing her.

In a subsequent statement issued to the press, Biden indicated that Harris had his "full endorsement" and encouraged Democrats to unite behind her candidacy.

In a follow-up posted to X, Biden continued, "My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President. 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."

This is a developing story and will be updated as events warrant.