Former US Secretary of State Hillary Clinton endorsed presumptive Democratic candidate Joe Biden's 2020 White House bid Tuesday, saying she is "thrilled" to join the campaign.

"Just think of what a difference it would make right now if we had a president who not only listened to the science, put facts over fiction, but brought us together, showed the kind of compassion and caring that we need from our president and which Joe Biden has been exemplifying throughout his entire life," Clinton said during a livestream with Biden.

"Think of what it would mean if we had a real president, not just somebody who plays one on TV," she added.

Biden and Clinton served in the administration of former President Barack Obama, and the previous president lent his support to his number two earlier this month after staying out of the Democratic White House race until Biden was the sole contender.

Independent Senator Bernie Sanders was Biden's sole challenger heading into April, but bowed out and endorsed the former vice president one day prior to Obama's announcement.

In so doing, Sanders, who draws widespread support among progressive Americans, urged a united Democratic front against President Donald Trump.

Indeed, it was Sanders whose campaign galvanized droves of new voters who sought an alternative to the Democratic establishment, and he remains popular among young voters, Latinos and within left-leaning circles, even as his campaign was dealt significant setbacks at the polls.

Whether those voters will lend their support to Biden remains to be seen. But Sanders is hoping they will.

Biden's candidacy will not be formalized until this summer's Democratic convention, but given the coronavirus pandemic it is unclear if the meeting will be held as intended in August, or will be held virtually as Biden has proposed. It has already been pushed back until August.

The pandemic has virtually halted all in-person campaigning ahead of November's polls.