The presidential race changes abruptly less than four months before Election Day. Joe Biden, 81, dropped his campaign to stay in the White House on Sunday and endorsed Vice President Kamala Harris, 59, to replace him atop the ticket running against Donald Trump and Sen. J.D. Vance of Ohio. "I believe it is in the best interest of my party and the country for me to stand down and focus entirely on fulfilling my duties as president for the remainder of my term," the president says in a one-page statement posted on social media. For her part, Harris says in a statement: "My intention is to earn and win this nomination. I will do everything in my power to unite the Democratic Party — and unite our nation – to defeat Donald Trump."

No U.S. president has quit a race this late. Biden did so after pressure from close allies amid worrisome poll numbers and concerns that he's too old and frail to beat former President Trump. Worries surfaced after Biden performed weakly in a June 27 televised debate.

If she's nominated at the Democratic National Convention in Chicago next month and wins Nov. 5, Harris would be the first female president. The vice president, a former California attorney general, earns wide support from elected Democrats and influential party figures, including Bill and Hillary Clinton (a past president and secretary of state). She inherits over $240 million in the Biden-Harris campaign donations and other support. But her party must navigate a shift that's unprecedented this late in an election year.

The vice president faces potential hurdles in a situation that's wide open and starts with her choice of a running mate who'll also need approval from a majority of 4,700 delegates at the Aug. 19-22 convention. Biden can't give his roughly 3,900 delegates, won in state primaries and caucuses, to Harris -- though his endorsement is influential. In Chicago, delegates can vote for anyone they like, so drama could arise. In any event, discussions about age now focus on Trump, 78, the oldest candidate to run for president.

Biden says: "It's time to come together and beat Trump. Let's do this." – Sunday post on social media

GOP congressional leader says: "If Joe Biden is not fit to run for President, he is not fit to serve as President. He must resign the office immediately." – House Speaker Mike Johnson, R-Louisiana

Democratic figure says: "Now that the donors and elected [officials] have pushed out the only candidate who has ever beaten Trump, it's time to end the political fantasy games and unite behind the only veteran of a national campaign — Kamala Harris" – Ron Klain, former Biden chief of staff