Warren had finished well behind the two front-runners on Tuesday in 14 states, including her home state of Massachusetts, leaving her path to the nomination virtually nonexistent.

Her exit ensures that what was once hailed as the most diverse field of candidates in US history has narrowed to a choice between two white, septuagenarian men.

Warren, who still commands a loyal base of supporters, did not immediately endorse either of her rivals, saying she would decide at a later time whether to do so.

Biden, a 77-year-old moderate, and Sanders, a 78-year-old liberal from Vermont, have emerged as the standard-bearers for the two major wings of the Democratic Party.

Outside her home in Cambridge, Massachusetts, on Thursday, Warren spoke bluntly about her failure to find a middle ground between the party's duelling factions.

"I was told when I first got into this, there are two lanes," she said. "I thought it was possible that wasn't the case, and there was more room to run a different kind of campaign. Apparently that wasn't the case."

The notion of "electability" meant Democrats prioritised defeating Republican President Donald Trump over all other concerns, seemed especially damaging for Warren and other non-white male candidates.

"The general narrative was that the women might be too risky, and I think there were people who heard that enough that it started showing up in polling ... and becomes a vicious cycle that was hard to break out of," said Christina Reynolds, vice president of communications at EMILY's List, which works to elect women supporting abortion rights and had endorsed Warren.

Asked on Thursday about the role that gender played in the campaign, Warren said it was a tricky issue for female candidates to address.

"That is the trap question for every woman," she said. "If you say, 'Yeah, there was sexism in this race,' everyone says, 'Whiner!' If you say, 'No, there was no sexism,' about a bazillion women say, 'What planet do you live on?'"

US Representative Tulsi Gabbard of Hawaii remains in the race, but has repeatedly failed to win even 1 per cent of the vote in primaries.

In Washington, Democratic US House of Representatives Speaker Nancy Pelosi said she believes the country is ready for a woman president but said misogyny still plays a role.

"Every time I get introduced as the most powerful woman or whatever, I almost cry because I think - I wish that were not true," she said.

Australian Associated Press