"Our campaign is not only about defeating Donald Trump," the 77-year-old self-described democratic socialist said in an email.

"Our campaign is about transforming our country and creating a government based on the principles of economic, social, racial and environmental justice."

Sanders stunned the Democratic establishment in 2016 with his spirited challenge to Hillary Clinton. While she ultimately became the party's nominee, his campaign helped lay the groundwork for the leftward lurch that has dominated Democratic politics in the Trump era.

He won more than 13 million votes in 2016 and dozens of primaries and caucuses, and opens his campaign with a nationwide organisation and a proven small-dollar fundraising effort.

The campaign said it had raised more than $US4 million in the 12 hours since announcing the 2020 tilt, including from 150,000 individuals.

The question now for Sanders is whether he can stand out in a crowded field of Democratic presidential candidates who also embrace many of his policy ideas and are newer to the national political stage.

While Sanders had been working to lay the groundwork for a second campaign for months, it was unclear whether he will be able to expand his appeal beyond his largely white base of supporters.

In 2016, Sanders notably struggled to garner support from black voters, an issue that could become particularly pervasive during the primary race.

Sanders also faces different pressures in the MeToo era. Some of his male staffers and supporters in 2016 were described as "Bernie bros" for their treatment of women.

In the run-up to Sanders' 2020 announcement, persistent allegations emerged of sexual harassment of women by male staffers during his 2016 campaign.

Australian Associated Press