Former US vice president Joe Biden has officially joined the Democratic presidential contest to replace Donald Trump in 2020.

The 76-year-old lifelong politician becomes an instant front-runner alongside Bernie Sanders, who is leading many polls.

In his campaign video, released on Thursday morning, Mr Biden warned that handing Mr Trump a further four years in the White House would "forever and fundamentally alter the character of this nation".

He also attacked the president for his response to a white supremacist rally in Charlottesville, Virginia, in 2017, accusing Mr Trump of having "assigned moral equivalence between those spreading hate and those with the courage to stand against it".

“In that moment, I knew that the threat to this nation was unlike any I had ever seen in my lifetime,” he added.

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

“We are in the battle for the soul of this nation.

"I believe history will look back on four years of this president, and all he embraces, as an abhorrent moment in time, but if we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation, who we are. And I cannot stand by and watch that happen.

"The core values of this nation, our standing in the world, our very democracy; everything that has made America America is at stake."

Joe Biden: 'Our leadership is giving license to this prejudice'

The announcement, made in a video posted on Twitter, marks the unofficial end of the chaotic early phase of the 2020 presidential season.

At least 20 Democrats have put their names in the hat for the chance to take on the incumbent Republican president next year, while several lesser-known candidates may still join the race.

Joe Biden and Barack Obama through the years Show all 15 1 /15 Joe Biden and Barack Obama through the years Joe Biden and Barack Obama through the years Joe Biden and Dr Jill Biden watch Barack Obama's farewell speech on 11 January. Obama called Biden his 'brother' Joe Biden and Barack Obama through the years US President Barack Obama speaks alongside US Vice President Joe Biden about the Affordable Care Act AFP/Getty Images Joe Biden and Barack Obama through the years Vice President Joe Biden and President Barack Obama Getty Joe Biden and Barack Obama through the years President Obama listens to Joe Biden speak of his work on defeating cancer on 18 October in the White House Reuters Joe Biden and Barack Obama through the years U.S. President Barack Obama is applauded by House Speaker Paul Ryan and Vice President Joe Biden while delivering his final State of the Union address to a joint session of Congress in Washington Reuters Joe Biden and Barack Obama through the years U.S. Vice President Joe Biden interjects as President Barack Obama delivers remarks at a reception for the 25th anniversary of the White House Initiative on Educational Excellence for Hispanics at the White House in Washington REUTERS Joe Biden and Barack Obama through the years Obama and Vice President Joe Biden react after a heckler was removed for their extended interruption (Reuters) Joe Biden and Barack Obama through the years U.S. President Barack Obama shakes hands with Speaker of the House John Boehner (R) as Vice President Joe Biden looks on Joe Biden and Barack Obama through the years Barack and Michelle Obama and Vice-President Joe Biden observing a moment of silence outside the White House to mark the 13th anniversary of the 9/11 attacks Getty Images Joe Biden and Barack Obama through the years Barack Obama and Joe Biden putt on the White House putting green Getty Joe Biden and Barack Obama through the years President Barack Obama and Joe Biden in April 2013 AFP/Getty Images Joe Biden and Barack Obama through the years January 1, 2013: U.S. President Barack Obama winks as he arrives with Vice President Joe Biden (L) in the briefing room Reuters Joe Biden and Barack Obama through the years President Barack Obama, Vice President Joe Biden and others receive an update on the mission against Osama bin Laden in the Situation Room of the White House May 1, 2011 in Washington, DC Getty Images Joe Biden and Barack Obama through the years Vice-President Joe Biden, right, confirmed that the US was looking at ways of taking legal action against Julian Assange - back in December 2010 GETTY IMAGES Joe Biden and Barack Obama through the years Joe Biden, left, and retired military officers watch President Barack Obama sign orders to close down the detention centre at Guantanamo Bay, Cuba, in January 2009 GETTY IMAGES

"Amtrak Joe" – a moniker he earned for commuting daily to the US capital by rail during his time as senator – will hope to capitalise on his down-to-earth manner and blue-collar upbringing if he is to win the Democrat nomination.

He remains one of the most recognisable names in US politics, but as an older white man who spent a half-century in Washington, it is unclear if he will be embraced by an increasingly liberal Democratic Party.

He has also been dogged by questions about his past, including recent claims he touched women in an overly familiar manner without their consent. Mr Biden has pledged to be "much more mindful" of respecting personal space.

Four years Mr Trump's senior, Mr Biden – who has said he would campaign as an "Obama-Biden Democrat" – would be the oldest person ever elected president should he win.

Joe Biden had the best reaction when Obama awarded him the Presidential medal of freedom

The former vice president will be in the Pennsylvania, a state that swung to Mr Trump in 2016 after voting for Democratic presidential candidates for decades, three times within the opening weeks of his campaign, the Associated Press reported.

He will be in Philadelphia on Thursday evening headlining a fundraiser at the home of David L Cohen, executive senior vice president of Comcast, where he hopes to raise $500,000 (£388,000), and will return to the city in the next two weeks for a major rally.

The Republican Party responded to rumours of an imminent announcement by Mr Biden with a video released on Wednesday questioning economic growth during the Obama presidency and resurrecting arguments against the former president's health care law.