WASHINGTON • Former US vice-president Joe Biden, a moderate who has made his appeal to working-class voters who deserted the Democrats in 2016 a key part of his political identity, launched a bid for the White House yesterday as the party's instant front runner.

Mr Biden announced the third presidential bid of his career through video on YouTube and other social media. He is expected to make his first public appearance as a candidate on Monday at an event in Pittsburgh featuring union members, a key constituency.

In his video, Mr Biden drew a stark contrast between himself and President Donald Trump.

"Everything that has made America America is at stake," he said, adding: "I believe history will look back on four years of this president and all he embraces as an aberrant 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 let that happen."

Mr Biden, 76, had been wrestling for months over whether to run. His candidacy will face numerous questions, including whether he is too old and too centrist for a Democratic Party yearning for fresh faces and increasingly propelled by its more vocal liberal wing.

He has consistently led the Democratic primary polls, collecting between a quarter and a third of the vote, and he is expected to receive robust support from donors in the Democratic establishment and the national business community.

Critics say his standing in polls is largely a function of name recognition for the former US senator from Delaware, whose more than four decades in public service includes eight years as president Barack Obama's No. 2 in the White House.

Mr Obama's spokesman, Ms Katie Hill, said in a statement after Mr Biden's announcement that Mr Obama has long said selecting Mr Biden to be his running mate in 2008 was one of the best decisions he ever made.

As speculation about his bid mounted, Mr Biden faced new questions about his propensity for touching and kissing strangers at political events, with several women coming forward to say he had made them feel uncomfortable.

Mr Biden struggled in his response to the concerns, but ultimately apologised and said he recognised standards for personal conduct had evolved in the wake of the #MeToo movement.

Mr Trump and his allies seized on the flap, attempting to weaken the incumbent President's likely top rival before Mr Biden entered the race. Even so, Mr Biden was determined to push forward, arguing that his background, experience and resume best positioned him to take on Mr Trump next year.

In a speech to union members this month, Mr Biden called Mr Trump a "tragedy in two acts".

"This country can't afford more years of a president looking to settle personal scores," he said.

In his announcement yesterday, Mr Biden held up the example of the August 2017 attack at a white nationalist rally in Charlottesville, Virginia, that killed a female counter-protester as a defining moment for the nation.

"It was there... we saw (Ku Klux) Klansmen and white supremacists and neo-Nazis come out in the open... bearing the fangs of racism," Mr Biden said, criticising Mr Trump's remarks at the time that there were "very fine people on both sides".

There was no moral equivalence between racists and those fighting such inequality, he said. "In that moment, I knew the threat to this nation was unlike any I had ever seen in my lifetime," said Mr Biden.

Mr Biden is widely seen as a politician with an uncommon gift for empathy and for communicating with voters on an emotional level. Known for his verbal gaffes on the campaign trail, he failed to gain traction with voters during his previous runs in 1988 and 2008.

REUTERS, NYTIMES