US Senator Elizabeth Warren, struggling to move past criticism over her claims of Native American heritage, has aimed for a fresh start with the formal launch of her 2020 presidential campaign.

The Massachusetts Democrat, a leader of the party's progressive wing, made her announcement from a historic site in Lawrence, northwest of Boston, that launched the US organised labour movement.

Warren, 69, has made worker rights, fair wages and access to health care central to her campaign.

"This is the fight of our lives. The fight to build an America where dreams are possible, an America that works for everyone," Warren said on Saturday.

"And that is why I stand here today: to declare that I am a candidate for President of the United States of America."

Warren is part of an increasingly crowded and diverse field of Democrats vying for the chance to challenge President Donald Trump, the likely Republican nominee.

A year before any ballots are cast in a Democratic primary, many of those candidates are spending this weekend talking to voters in the early-nominating states of Iowa, New Hampshire and South Carolina.

Warren's announcement will be followed on Sunday by US Senator Amy Klobuchar, who has said she will reveal her presidential plans in her home state of Minnesota.

Warren picked up the endorsement of the Progressive Change Campaign Committee simultaneously with her launch. The group could pump millions of dollars behind her candidacy and provide an outside attack dog against her Democratic opponents.

"We believe that Elizabeth Warren is the most electable Democrat and the best person to be president to defeat Trump," Adam Green, the co-founder of PCCC, said.

Warren's heritage claims have dogged her since her first campaign for the Senate in 2012, and Trump mockingly refers to her as "Pocahontas".

Her ancestry drew fresh scrutiny earlier this week with the discovery that she described her race as American Indian on a form to join the Texas legal bar in the 1980s.

Warren has repeatedly apologised, saying the claim was based on "family lore" and she now understands tribal sovereignty dictates membership.

Trump's campaign described Warren as a "fraud". A statement from his campaign manager Brad Parscale said, "The American people will reject her dishonest campaign."

In addition to being a fierce Trump critic, Warren is an outspoken critic of Wall Street who gained national notoriety for her efforts to create the federal Consumer Financial Protection Bureau to regulate large banks.

Warren is one of four women so far seeking the Democratic nomination, an unprecedented number of female candidates vying to lead a country that has never had a woman chief executive.