11:18 AM PT -- Mere minutes after dropping out of the race, President Biden has fully endorsed Vice President Kamala Harris in the 2024 presidential race ... saying picking her as his number 2 was the best decision he's made -- and asking Dems to come together to defeat Donald Trump.

President Biden is waving the white flag on his reelection bid ... dropping out of the race for the White House amid mounting pressure from within his own party.

POTUS announced Sunday he will not be at the top of the Democratic ticket in 2024 in a lengthy statement published to X ... saying it's in the best interests of the nation for him to step aside.

In the statement, Biden touts his administration's accomplishments ... citing a reivatalized economy, lower prescription drug costs and passed gun safety laws among other notable achievements.

He thanks the American people for their hard work in building a better, stronger nation and calls the presidency the honor of his life ... before saying he won't run for reelection.

Biden promises to speak to the nation later this week to explain his decision in more depth ... before thanking Vice President Kamala Harris for her partnership during his administration.

Biden recently said only a medical condition would get him to leave the race ... and then he tested positive for COVID-19.

Still, Biden's exit felt almost inevitable after his disastrous debate performance back in June and concerns about his age and mental gaffes ... with the drum beat for him to leave getting louder ever since.

Top Democrats had been calling on Biden to drop out ... as well as some of the party's most famous celebrity supporters, like George Clooney.

As we told you ... big-time Democratic donors were withholding money for Biden's presidential campaign ... and we got video of ABC anchor George Stephanopoulos telling a fellow New Yorker he didn't think Biden could serve another 4 years.

Biden was defiant for several weeks, trying to save a sinking ship ... but in the end, he finally saw the writing on the wall.

Now, the big question ... who will the Democrats nominate for president and vice president???

November is fast approaching so the Dems will have to move quickly here if they want to defeat Donald Trump at the ballot box.

Bottom line ... Biden's going down as a one-term president.