US President Joe Biden has announced that he will end his candidacy for re-election, saying “it is in the best interest of my party and the country”.

It comes four months before Americans go to the polls, upending the race for the White House. It follows weeks of intense pressure from fellow Democrats after a faltering debate performance against Republican Donald Trump at the end of June.

Following the announcement yesterday evening, President Biden gave his endorsement in the presidential race to Vice President Kamala Harris. In a letter posted to his social media account, he said it had been the greatest honour of his life to serve as president.

“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 to focus solely on fulfilling the duties as President for the remainder of my term”.

He said in his statement he would address the nation on the matter later in the week, reports the BBC. Calls for Biden to withdraw from the race began to grow after a disastrous debate performance in late June against Donald Trump.

During the debate he was criticised for often being incoherent and speaking in a weak voice – something which his party claimed as down to a cold. Following it, there were growing calls from within his own party for him to stand aside, with a growing number of congressional Democrats worried his campaign would hurt their own re-election chances in November.

A handful of senators, who sit in the upper chamber of Congress, also called for him to withdraw. In his statement, President Biden thanked his Vice President Kamala Harris, saying she was an “extraordinary partner”. “And let me express my heartfelt appreciation for the American people for the faith and trust you have placed in me,” his statement added.

“I believe today and always have: that there is nothing America can’t do – when we do it together. We just have to remember we are the United States of America.” Last week he returned to his home in Delaware after being diagnosed with Covid, but said on Friday he was looking forward to “getting back on the campaign trail next week”.

He has previously said only the “Lord Almighty” could make him withdraw, but then later said he would consider withdrawing if he had a health condition. Democrats immediately praised his decision. “Joe Biden has not only been a great president and a great legislative leader but he is a truly amazing human being.

His decision of course was not easy, but he once again put his country, his party, and our future first,” said Chuck Schumer – the majority leader in the US senate, and one of several Democrats who had been pressuring Biden to step aside – in a statement. “Joe, today shows you are a true patriot and great American.”

Tammy Baldwin, the Wisconsin Democrat seeking re-election in a competitive US senate race, said: “It has been an honour to work with Joe Biden to deliver real, meaningful change for working Wisconsinites across our state … throughout all of that work, I’ve been inspired by his decency, integrity, and dedication to service, and I am deeply grateful for that.

Thank you, President Biden.” Minutes after yesterday’s announcement, Trump told CNN that in his opinion the president had been “the single worst president by far in the history of our country”. Trump also told the network he thinks it is going to be easier to defeat Harris than it would have been to beat Biden.

Elise Stefanik, one of the top Republicans in the US House, said Biden should resign the presidency. JD Vance, Trump’s running mate, also called on Biden to leave office yesterday. “If Joe Biden can’t run for reelection, he is unable and unfit to serve as president of the United States. He must immediately resign,” she said in a statement.

“The Democrat party is in absolute free fall for their blatantly corrupt and desperate attempt to cover up the fact that Joe Biden is unfit for office.” Biden’s decision to step aside from the race, though remaining as president, caps a singular few weeks in American politics, the latest stunning episode in an unusually tumultuous election season.

Trump, the former president and Republican nominee, narrowly survived an attempt on his life during a campaign rally in Pennsylvania that bloodied his ear and left one spectator dead. Biden, after appealing for calm in the wake of the attack, had returned to the campaign trail last week determined to salvage his candidacy and once again prove his doubters wrong.

In media appearances, the president was defiant, insisting that he would remain the party’s standard-bearer in November, barring an intervention from the “Lord Almighty”, being struck by a train or a medical condition. On Wednesday, as Biden was set to deliver remarks at a conference in Nevada, he tested positive for Covid.

The president’s withdrawal pushes the Democratic party into largely uncharted waters, with its national convention scheduled to begin on August 19 in Chicago. The nominee will also have a tight window to choose a running mate to take on Trump and his vice-presidential pick, the Ohio senator JD Vance.

It is not clear how Democrats will choose a new ticket. The 95% of delegates who pledged to support Biden following his big wins in the Democratic primaries are now able to vote for a different candidate. Roughly 4,000 Democratic delegates will convene next month to choose a new nominee, and Kamala Harris will arrive in Chicago as an early favourite in the race to replace Biden.

After serving as Biden’s vice president, Harris, 59, has the largest national profile of any Democratic candidate, and delegates may view her as the safest option with just four months to spare before election day. Campaign finance experts also say that Harris would have the most straightforward legal argument to keep the Biden campaign’s fundraising haul, while another nominee may have to forfeit that money.

As of the end of May, the Biden campaign had $91.6m in cash on hand. Despite Harris’s advantages, her nomination is not automatic, and other lawmakers – including California Governor, Gavin Newsom, Michigan Governor, Gretchen Whitmer and Illinois Governor, JB Pritzker – have been named as potential alternatives.

If any of those candidates were nominated in Chicago next month, they would face the monumental task of introducing themselves to voters, crafting a campaign message and defeating Trump all in two-and-a-half months.

Yet many Democrats prefer to risk the unknown than stand behind a nominee of whom nearly two-thirds of his own supporters said should quit the race, according to an AP-Norc Centre for Public Affairs Research survey released on Wednesday.

In an interview with BET last week, Biden indicated that he had initially expected to serve one term, as many voters expected, recalling a pledge he made during the 2020 campaign to be a “bridge” to the next generation of Democratic leaders. “I was going to be a transitional candidate, and I thought I would be able to move on from this and pass it on to somebody else,” the president told BET.

“But I didn’t anticipate things getting so, so, so divided.” Still, in recent days, Biden had stepped up the praise of his vice-president, emphasising her readiness to serve. “She’s not only a great vice president,” Biden said in remarks last week at the NAACP convention in Las Vegas, “she could be president of the United States.”