Washington, United States -Vice President Joe Biden said  he will not run for the White House in 2016, ending months of speculation that has stalked Hillary Clinton's campaign and threatened Democratic party unity.

Pointing to the tyranny of America's electoral calendar, the two-time presidential hopeful reluctantly relinquished a long-held political dream and in doing so removed a potentially sizable obstacle to Clinton's nomination.

"I believe we're out of time, the time necessary to mount a winning campaign for the nomination," Biden said in an emotion-tinged statement at the White House, flanked by his wife Jill and the man he had hoped to replace, President Barack Obama. 

Biden's months of soul searching and deliberation have coincided with mourning for his late son Beau, who succumbed to brain cancer in May.

In 1972, weeks after being elected a lawmaker, Biden lost his wife and baby daughter in a car crash that also left his two young sons badly injured.

Biden took his Senate oath of office at Beau and Hunter's hospital bedside.

Today -- standing in the sun-bleached Rose Garden, within touching distance of the Oval Office and the Resolute Desk -- Biden said that thoughts of Beau now brought a smile to the lips before a tear to the eye, but it was too late.

With just over two months until voters in Iowa cast the first ballots of the 2016 primary campaign, the 72-year-old may have struggled to garner enough cash, endorsements and the campaign team to rival the Clinton juggernaut.

The effort to draft Biden had been fueled by disillusionment with Clinton, who has struggled to gain traction among voters and remains a deeply polarizing figure.

Responding to the news, Clinton in a statement described Biden as "a good friend and a great man." 

"Today and always, inspired by his optimism and commitment to change the world for the better," she said on Twitter.

While he may not be running, Biden made clear that he intends to play the party elder.

"While I am not going to be a candidate, I will speak out clearly and forcefully as to where we stand as a party and where we need to go as a nation," he said.

But with obvious strains of regret, Biden sketched out what a working-class kid from Pennsylvania would have done as the occupant of 1600 Pennsylvania Avenue.

First, he said, was a "moon shot" to cure cancer.

"I would have wanted to have been the president that ended cancer."

"It is personal," he said, adding: "I know we can do this... because there are so many breakthroughs just on the horizon in science and medicine."

- Warning shots - 

But there were also words of warning for those who have opposed key parts of Obama's legacy and reveled in arm-wrestling with Republicans -- comments likely to be seen as targeted at Clinton.

Biden implored Democrats and Republicans to "end the divisive partisan politics that is ripping this country apart."

"It is mean-spirited and petty and it has gone on for much too long," he said.

"I don't think that we should look at Republicans as our enemies. They are opposition. They are not our enemies. And for the sake of the country, we have to work together."

Where Obama has been criticized for remaining aloof from the backroom dealing of Washington politics, his deputy has relished getting his hands dirty, flashing his pearly whites, and forging unlikely alliances.

He cultivated personal relationships with Republicans during his decades in the US Senate.

Addressing Democrats, Biden added that "our nation will be making a tragic mistake if we walk away or attempt to undo the Obama legacy.

"Democrats should not only defend and protect this record, but they should run on the record," he said.

Clinton recently announced her opposition to a Pacific free-trade deal she supported while serving as Obama's secretary of state. 

The man who is currently Clinton's nearest rival, Vermont Senator Bernie Sanders, also took to Twitter to praise Biden.

"I thank @VP for his continued service to the nation and his support of the middle class," he said.

The Republican National Committee's chairman, Reince Priebus, meanwhile called Biden "the most formidable general election candidate the Democratic Party could have fielded."

Biden's no-go "greatly improves our chances of taking back the White House," he added.