US President Joe Biden dropped out of the presidential race Sunday as his poor debate performance last month exacerbated concerns about his old age and touched off unrelenting calls to end his reelection campaign.

Biden, 81, made the announcement, bowing to mounting pressure from Democratic lawmakers, donors and voters. He officially endorsed Vice President Kamala Harris to become the Democratic Party's nominee to take on former President Donald Trump in the Nov. 5 general election.

It is unprecedented in America's modern history that despite dominating monthslong nomination contests, a presidential candidate suspended his campaign just weeks before a national party convention. Since the early 1950s, two Democratic presidents -- Harry Truman and Lyndon Johnson -- have desisted from running for another term during an election year.

The Democratic Party has been preparing to hold its national convention in Chicago, Illinois, from Aug. 19-22 to officially nominate its standard-bearer. The Republican Party formally crowned Trump as the GOP nominee during its convention in Milwaukee, Wisconsin, last week.

"It has been the greatest honor of my life to serve as your president," Biden said in a statement posted on X, formerly Twitter.

"And while it has been my intention to seek re-election, 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," he added.

In a separate post, Biden officially expressed his support for Harris.

"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," he said.

"Today, I want to offer my full support and endorsement for Kamala to be the nominee of our party this year. Democrats -- it's time to come together and beat Trump. Let's do this."

His exit from the race came amid unrelenting pressure on him to step aside following the June 27 presidential debate where he lost his train of thought, misspoke and failed to articulate his views on certain issues while Trump ramped up his criticism of Biden's policy.

Dismissing the woeful debate performance as a one-off episode and stressing age as a source of "wisdom," Biden had been trying to squelch concerns about his age through media interviews, a press conference and other public appearances. But the efforts failed to quell calls from Democratic leaders and others for him to bow out.

Some party members not only questioned the viability of Biden's candidacy but also voiced concerns over the possibility of its impact on down-ballot races.

Harris, 59, vowed to do "everything" to bring the Democratic Party together to defeat Trump in the general election.

"I am honored to have the president's endorsement, and my intention is to earn and win this nomination. Over the past year, I have traveled across the country, talking with Americans about the clear choice in this momentous election. And that is what I will continue to do in the days and weeks ahead," she said in a statement carried by the New York Times.

"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. We have 107 days until Election Day. Together, we will fight. And together, we will win."

Despite Biden's endorsement for Harris, uncertainty still lingers over whether she can be officially nominated given that more than 4,600 party delegates are free to vote for any candidate and will determine the ultimate nominee.

But in the event of a contested nomination process, Harris -- with the backing of Biden -- is seen as being at an advantage, considering that Biden's campaign could help her with a growing number of Democrats and party supporters starting to coalesce behind her.

A former senator from California, Harris, America's first female vice president, is expected to represent a new generation in the election battle against Trump, 78. If formally nominated, she would get an opportunity to try to become the first US woman president.

Soon after Biden's announcement, Trump told CNN that he thinks Harris will be easier to defeat than Biden would have been. He also criticized Biden as one that goes "down as the single worst president by far in the history of our country."

In a post on Truth Social, Trump continued his criticism against Biden, saying the incumbent president is "not fit to run for president" and "not fit to serve."

"He only attained the position of president by lies, fake news, and not leaving his basement," he wrote. "All those around him, including his doctor and the media, knew that he wasn't capable of being president, and he wasn't."

Biden has been self-isolating due to COVID-19 infection -- a pause that came at a time when questions have persisted over whether he is mentally and physically strong enough to effectively lead the country for another four-year term.

On July 15, Trump was officially nominated as the GOP nominee for the presidential election.

At the convention, his supporters cast him as a "resilient" leader as he has survived an assassination attempt during a recent campaign rally in Pennsylvania. The GOP event underscored his stable political footing despite his conviction in a high-profile hush-money trial in May.

In a statement, former President Barack Obama backed Biden's decision to opt out, describing the president as a "patriot of the highest order."

"I also know Joe 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," he said. "But I know he wouldn't make this decision unless he believed it was right for America."

Obama expressed "extraordinary confidence" that Democratic Party leaders will be able to "create a process from which an outstanding nominee emerges," while acknowledging the party will be navigating "uncharted waters" in the days ahead.

In a social media post, House Speaker Mike Johnson called for Biden's resignation.

"If Joe Biden is not fit to run for President, he is not fit to serve as President," the Republican speaker wrote on X. "He must resign the office immediately. November 5 cannot arrive soon enough."

He also issued a warning against potential US "adversaries."

"Regardless of the chaos in the current White House, our adversaries around the globe should be reminded that the US Congress, the US military, and the American people are fully prepared and committed to defend our interests both at home and abroad," he said. (Yonhap)