Joe Biden has received the endorsement of his last remaining challenger Bernie Sanders, quelling fears of continuing Democratic division.
Mr Sanders encouraged his progressive supporters to rally behind the presumptive Democratic nominee, to help him defeat Donald Trump.
The Vermont senator had dropped out of the race to take on the president in November’s election on 8 April, but was yet to officially offer his support to the former vice president.
He told Mr Biden, in a virtual event: “I am asking all Americans, I’m asking every Democrat, I’m asking every independent, I’m asking a lot of Republicans, to come together in this campaign to support your candidacy, which I endorse.”
The presidential campaign of Mr Sanders – who is usually seen as being on the left of his party – was centred around policies such as universal health care.
The former vice president must now bridge the ideological divides which have become entrenched in the Democratic Party to put together a coalition that can beat the incumbent president.
Disunity helped Mr Trump beat Hillary Clinton in 2016.
Mr Sanders ended up backing Mrs Clinton four years ago, but only after a drawn-out nomination fight that only ended with the summer convention.
Appearing on the same screen as Mr Biden, the veteran senator said there’s “no great secret out there that you and I have our differences,” but, he added: “We’ve got to make Trump a one-term president… I will do all that I can to make that happen.”
Both candidates have been unable to appear together in person because of the coronavirus, which has hampered normal campaigning in the latter stages of the primaries.
Before they got under way in January, more than a dozen Democrats were vying for the nomination.
Many dropped out in the early rounds, leaving Mr Biden unopposed months before the convention which was originally scheduled to be held in July but has now been postponed to August.
Mr Biden, 77, who is usually viewed as having more moderate policies, has already made some overtures to progressives by embracing aspects of Mr Sanders’ policies and those of another from the left, Senator Elizabeth Warren.
Divisions remain on show however.
On Monday, New York Representative Alexandria Ocasio-Cortez, who many have tipped as a potential presidential candidate in 2024, criticised Mr Biden’s adoption of some progressive health care policies.
“We need a real plan and not just gestures,” she said.
Mr Trump’s allies, meanwhile, took the opportunity to go on the attack.
The president’s campaign manager Brad Parscale said that “though Bernie Sanders won’t be on the ballot in November, his issues will be”.
“Biden had to adopt most of Bernie’s agenda to be successful in the Democrat primaries,” he added.