Former President Barack Obama stepped from the political sidelines on Tuesday and formally endorsed his former Vice President, Joe Biden, giving the presumptive Democratic presidential nominee a boost from the party's biggest fundraiser and one of its most popular figures.

In a 12-minute video released by the Biden campaign, Obama said Biden has "the character and the experience to guide us through one of our darkest times and heal us through a long recovery" and would, if elected president, surround himself with people "who actually know how to run the government and care about doing a good job running the government".

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Without mentioning President Donald Trump by name, Obama said strong leadership - leadership guided by honesty, humility, empathy and grace - needed to come from the White House and not just the state houses and mayor's offices around the country.

"This crisis has reminded us that government matters," Obama said. "It's reminded us that good government matters. That facts and science matter. That the rule of law matters. That having leaders who are informed and honest, and seek to bring people together rather than drive them apart - those kind of leaders matter."

I’m proud to endorse my friend @JoeBiden for President of the United States. Let's go: https://t.co/maHVGRozkX — Barack Obama (@BarackObama) April 14, 2020

Democratic strategist Antjuan Seawright called the endorsement a "game changer" for Biden.

"He is a uniting partner in helping bridge this party after an intense primary and paving the road ahead," Seawright said of the former president.

Obama and Biden are known to be close friends from their two terms in the White House, and Biden leaned heavily on his affiliation with the former president throughout the Democratic primary, touting their relationship and framing his pitch as an extension of Obama's presidency.

But Obama stayed above the fray in the primary, rarely speaking out about the intra-party fight. The former president offered his private counsel to any Democratic presidential contender who asked for it, but made no efforts to bolster any single candidate's campaign — including Biden's, despite their long history.

In a statement released by President Trump's campaign, campaign manager Brad Parscale said Obama's decision to endorse Biden was born from necessity and not enthusiasm.

"Barack Obama spent much of the past five years urging Joe Biden not to run for president out of fear he would embarrass himself," Parscale said. "Now that Biden is the only candidate left in the Democratic field, Obama has no other choice but to support him. Even Bernie Sanders beat him to it. Obama was right in the first place: Biden is a bad candidate who will embarrass himself and his party.

"President Trump will destroy him," Parscale added.

On Monday, Bernie Sanders endorsed his former rival for the Democratic nomination and encouraged his progressive supporters to rally behind him in an urgent bid to defeat President Donald Trump in the November general election. Media reports in the US suggest that several phone calls between Obama and Sanders in recent weeks played a role in his decision to withdraw from the race and endorse Biden.

Obama spent a significant portion of Tuesday's Biden endorsement video praising Sanders, calling him "an American original".

"The ideas he's championed; the energy and enthusiasm he inspired - especially in young people - will be critical in moving America in a direction of progress and hope," Obama said.