Joe Biden: Former US Vice President to announce third run for White House today

Joe Biden decided against a 2016 presidential bid while mourning his son Beau. (Courtesy)

Former Vice President Joe Biden is set to launch his third run for the White House today.

Biden is due to finally announce his bid by video followed by his first public appearance as a candidate on Monday at an event in Pittsburgh, a key constituency.

A moderate Democrat, Biden wants to appeal to the disaffected working-class voters who deserted the party in 2016 for Donald Trump.

The 76-year-old's candidacy is expected to face a number of questions though, including whether he is too old and too centrist for the Democratic Party.

Biden has also faced new questions about his longtime habit of touching and kissing strangers at political events, with several women coming forward to say he had made them feel uncomfortable.

The septuagenarian struggled in his response to the concerns, at times joking about his behaviour.

Trump and his allies seized on the flap, attempting to weaken his top rival before Biden entered the race.

He and the president have already clashed, even before his candidacy was announced.

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

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

Last year, he said he would "beat the hell" out of Trump if the two were in high school because of the way the president has talked about women.

That prompted Trump to call him "Crazy Joe Biden" and claim on Twitter that Biden would "go down fast and hard, crying all the way" if they fought.

Biden later regretted the exchange, saying "I shouldn't get down in the mosh pit with this guy."

Known for his verbal gaffes on the campaign trail, Biden failed to gain traction with voters during his previous runs in 1988 and 2008.

He dropped his 1988 bid amid allegations of plagiarisation.

But his experience and strong debate performances in 2008 impressed Barack Obama enough that he tapped Biden as his running mate.

Biden decided against a 2016 presidential bid while mourning his son Beau, who died of brain cancer in May 2015. His son had urged him to run.

His candidacy will offer early hints about whether Democrats are more interested in finding a centrist who can win over the white working-class voters who went for Trump in 2016, or someone who can fire up the party's diverse progressive wing.

He starts as the leader of the pack in opinion polls of a Democratic field that now will total 20 contenders seeking the chance to challenge the Republican president in November 2020.

Other include Senators Kamala Harris of California, Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts.