Encouraged by Democrats seeking an alternative to Clinton, Biden had spent the past several months deeply engaged in discussions with his family and advisers but has finally decided not to run for president.

Vice-president Joe Biden ended months of speculation on Wednesday about whether he would run for president, appearing in a hastily announced White House address to say that his window of opportunity to “mount a winning campaign for the nomination” had closed.

Biden cast the decision in personal terms, saying that his family had only recently regained its feet after the death of his eldest son, Beau, from cancer in May.

“As my family and I have worked through the grieving process,” he said, “I’ve said all along that it may very well be that that process, by the time we get through it, closes the window on mounting a realistic campaign for president.

“I’ve concluded it has closed.”

The announcement marked a major shift in the presidential race, removing a potential threat to Democratic front runner Hillary Clinton, who had been polling ahead of a hypothetical Biden candidacy but who was sure to lose support should the vice-president jump in.

It raised fears among White House loyalists that the legacy of the Obama administration had taken on a new vulnerability. And it added clarity for the career of the vice-president, 72, who was first elected to the Senate from Delaware in 1973 and whose time in the public eye has included two runs at the presidency, chairmanship of the Senate foreign relations committee, an unusually visible profile as the president’s No 2 and a public affection that has warmed over his seven years in the White House.

In an address announced about 10 minutes before he began speaking, a subdued Biden thanked the president for lending him the Rose Garden and spoke in personal terms about the loss of his eldest son to cancer in May.

“My family has suffered a loss, and I hope there will come a time … that sooner rather than later, when you think of your loved one, it brings a smile to your lips before it brings a tear to your eyes.

“That’s where the Bidens are today.”

The vice-president called on the other Democratic candidates not to run from the president’s record and received warm applause from officials as the pair walked slowly back to the Oval Office.

“This party, our nation, will make a tragic mistake if we walk away or attempt to undo the Obama legacy,” Biden said. “Democrats should not only protect this record, or defend this record – they should run on this record.”

Despite the crisp autumn sunshine, much of the garden was in shadow throughout the longer-than-expected speech by Biden and when he finished, he received a hug from Obama that spoke volumes about sympathy felt in the White House for Biden.

But there was also a tangible sense of relief among many staff who were also known to be concerned about the bitter civil war that could have re-erupted should Biden face off against Clinton.

Biden did repeat a criticism he has made in recent days of a remark Clinton made at the presidential debate last week that Republicans were the “enemy”.

“I believe that we have to end the divisive partisan politics that is ripping this country apart, and I believe that we can,” Biden said. “I don’t think, as some do, that we should look at Republicans as our enemy. They’re the opposition, they’re not the enemy.”

Biden promised to be active during the campaign.

“I will not be silent. I will speak clearly and forcefully … on where we stand as a nation.”

In a statement, Clinton called Biden “a good man and a great vice-president”.

“Serving alongside him in the Senate and then the administration, I saw first-hand his passion for our country and our people. Like millions of others, I admire his devotion to family, his grace in grief, his grit and determination on behalf of the middle class, and his unyielding faith in America’s promise.”

Vermont senator Bernie Sanders, now Clinton’s closest rival for the Democratic nomination called the vice-president “a good friend” who “has made the decision that he feels is best for himself, his family and the country”.

The Republican frontrunner, Donald Trump, said Biden had made the “correct decision for him and his family”, adding that he “would rather run against Hillary because her record is so bad”.

The organizers behind Draft Biden 2016, which had sought to encourage the vice-president to join the race, said they were pleased with the work done by people on his behalf.

“While the vice-president has decided not to run, we know that over the next year he will stand up for all Americans and articulate a vision for America’s future that will leave no one behind.”

“It is what it is and we’re left with candidates that we’re left with,” said Jon Cooper, the national finance chair at Draft Biden 2016, now a “superpac without a candidate”, as he described it.

While disappointed the vice president is not running, Cooper said he was relieved to hear Biden say he would continue to make his voice heard during the campaign.

“Hearing Joe Biden speak in the Rose Garden today made me realize even more why all of us undertook this effort in the first place and his speech reminded me of exactly why I and so many others were hoping he would enter the race,” Cooper said. “It was his honesty and his authenticity. His eloquence and passion and love for our country. I also thought he was the one candidate best positioned to restore some civility in Washington.”

‘We can do so much more’
The hastily arranged event was notable most strikingly for its timing: less than 24 hours before an appearance by Clinton before a House select committee on the Benghazi attacks that could prove a turning point in her own campaign.

By squeezing in the announcement just before Obama’s helicopter took off for a long-scheduled trip to West Virginia, the vice-president effectively chose the last possible moment to announce his decision without it looking like it was in reaction to Clinton’s performance.

Biden listed policy priorities he would support in the upcoming campaign, including upward economic mobility for the middle class, more funding for education and the need for coalition building in foreign policy.

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“The argument that we just have to do something when bad people do bad things isn’t enough,” Biden said, in apparent reference to the Russian presence in Syria and Ukraine.

He also called, passionately, for a “national commitment to end cancer as we know it today” and vowed to fight toward the goal in his remaining time in office.

“I believe we need a moonshot in this country to cure cancer,” he said. “It’s personal.”

As a candidate, Biden might have been able to pick off supporters from Clinton or Sanders, but he seemed to have no natural constituency – no collection of voters as grouped by race, sex, education, region or income – that was not just as well served by one of the other candidates.

Only 17% of Democrats in a recent Reuters/Ipsos poll said Biden would be their top pick as president, compared to 46% for Clinton. Almost half of Democrats said they wanted the vice-president in the race, however.

Biden has run for president twice before, with both bids ending early and poorly. He would seem to have retained some of his old weaknesses as a candidate, such as a tendency to go off message. In a new run, he would have brought old strengths too, however, including his formidable experience and ability as a retail campaigner.

Most significantly, for Biden backers, was the transformation they detected he had undergone since becoming vice-president in 2009. In the nearly seven years since he has become a widely loved national figure – if sometimes a figure of mirth – popular with his colleagues and respected on both sides of the aisle.

Biden had never cast his decision in terms of how well Clinton is faring, but as a question of whether the causes he has described as core – upward economic mobility and global stability – were being served.

On Wednesday, Biden said the core challenge facing the country was to relieve a feeling of familial doubt about the future.

“There are too many people in America, too many parents, who don’t believe they can look their children in the eye and say, ‘Honey, it’s going to be OK,’” he said.

“That’s our responsibility. And I believe it’s totally within our power.”

The vice-president concluded with a hug for the president and a kiss for his wife.

“I’m telling you,” he said, “we can do so much more, and I’m looking forward to working with this man to get it done.”