Sanders, 77, made the announcement in a radio interview in his home state of Vermont on Tuesday.
“I am running for president for two reasons,” said Sanders on Vermont Public Radio.
They are to oppose President Donald Trump and to enact many of the ideas that Sanders had championed in 2016, including universal healthcare access and the minimum hourly wage of $15, he said.
“I think the current occupant of the White House is an embarrassment to our country,” Sanders said.
“I think he is a pathological liar… I also think he is a racist, a sexist, a homophobe, a xenophobe, somebody who is gaining cheap political points by trying to pick on minorities, often undocumented immigrants.”
In an email to supporters early on Tuesday morning, Sanders pledged to build a vast grassroots movement to confront the special interests that he said dominate government and politics.
“Our campaign is about transforming our country and creating a government based on the principles of economic, social, racial and environmental justice,” he said.
The senator from Vermont launched his 2016 candidacy against Hillary Clinton as a long shot, but ended up capturing 23 state nominating contests and pushing the party to the left, generating tension between its establishment and liberal wings that has not entirely abated.
This time around, Sanders has been among the leaders in opinion polls of prospective 2020 candidates, but he faces a field more heavily populated with other liberal progressives touting many of the same ideas he brought into the party mainstream.
The field could also grow, with a number of high-profile Democrats still considering presidential bids, including former Vice President Joe Biden and former Texas Representative Beto O’Rourke.
The crowded field could make it harder for Sanders to generate the same level of fervent support as four years ago.
He is also likely to face questions about his age and relevance in a party that is increasingly advancing more diverse and fresh voices, including women and minorities – groups that Sanders struggled to win over in 2016.
The senator also faces different pressures in the #MeToo era.
In the run-up to Sanders’s 2020 announcement, persistent allegations emerged of sexual harassment of women by male staffers during his 2016 campaign. Politico and The New York Times reported several allegations of unwanted sexual advances and pay inequity.
Sanders offered an unequivocal apology over the complaints on January 10, saying: “What they [women] experienced was absolutely unacceptable and certainly not what a progressive campaign – or any campaign – should be about.”
The primaries and caucuses that determine the party’s nominee for next year’s election will begin in February 2020 in Iowa.
Sanders served in the House until 2006 when he was elected to the US Senate. He was re-elected in 2012 and 2018.
While Sanders remains popular among many Democrats, some in the party have questioned whether their champion this time around should be a septuagenarian white man.
Al Jazeera and news agencies