US Senator Elizabeth Warren has ended her campaign for the Democratic presidential nomination.

Ms Warren said she was not ready to back either of the remaining candidates - Joe Biden or Bernie Sanders.

"Not today," Ms Warren told reporters outside her home in Cambridge, Massachusetts, when asked if she was ready to endorse either of the two men.

"We don't have to decide that this minute," she said. "I want to take a little time to think a little more."

Ms Warren told staff that she was dropping out of the race after failing to win a single state on Super Tuesday.

She had emerged as a top Democratic contender for the White House on the strength of an anti-corruption platform backed by a dizzying array of policy proposals.

A former bankruptcy law professor who forged a national reputation as a scourge of Wall Street even before entering politics, Ms Warren had banked on a strong showing on Super Tuesday after a string of disappointing finishes in the early states.

But she trailed far behind front-runners Bernie Sanders and Joe Biden, placing third in her home state of Massachusetts.

Ms Warren's call with her staff came one day after former New York Mayor Michael Bloomberg, who also had a disappointing Super Tuesday, dropped out.

The 70-year-old was the last woman among the top tier of candidates in a Democratic field that began as the most diverse in history.

Her departure could provide a boost to Mr Sanders, a fellow liberal, if some of her support pivots to the senator from Vermont, though voters do not always follow ideological lines.

Six months ago Ms Warren was ascendant, having steadily risen in national opinion polls after dozens of deeply researched policy rollouts led to the slogan: "Warren has a plan for that."

She centered her campaign on battling the influence of money in politics, arguing that on every significant issue - gun control, climate change, healthcare - the power of corporate lobbyists and billionaires would stymie any opportunity to achieve meaningful change.

Many of Ms Warren's proposals drew from the lessons she learned from her bankruptcy research at Harvard University and her experience setting up the Consumer Financial Protection Bureau for then-President Barack Obama after the 2007-2009 financial crisis.

"The good news is, I have the most sweeping anti-corruption plan since Watergate," she was fond of saying on the campaign trail.

"The bad news is, we need the most sweeping anti-corruption plan since Watergate."

Like Mr Sanders, Ms Warren swore off high-dollar fundraisers and relied on grassroots donations to fuel her campaign.

She faced criticism last month for declining to disavow a new super PAC that boosted her campaign with an eight-figure advertising blitz, an apparent reversal of her longstanding opposition to outside spending.

Ms Warren's commitment to detailed plans prompted rivals to attack her for initially failing to explain how she would pay for Medicare for All, the sweeping healthcare overhaul also championed by Mr Sanders.

Her refusal to say at a televised debate in October whether she would raise middle-class taxes to finance it - even as Mr Sanders acknowledged he would do so while arguing that overall costs would decrease - drew criticism she was being evasive.

Weeks later, when she released a plan that raised enough revenue via taxing corporations and the wealthy, moderate candidates like Mr Biden, the former vice president, said the math did not add up.

Meanwhile, some progressives said her three-year, phase-in plan was backtracking.

Ms Warren often seemed caught in the middle of the Democratic battle for hearts and minds, unable to peel away enough liberal support from Mr Sanders or convince enough centrists that she could broaden her appeal.

She was also competing in a race in which Democratic voters desperate to defeat US President Donald Trump in November have endlessly debated the vague notion of her "electability," which some supporters saw as a thinly veiled way of questioning whether Americans are ready to elect a woman as president.

Her allies bristled that she faced a double standard due to her gender, noting that Mr Sanders - who spent months essentially waving off questions about his own lack of a detailed healthcare plan - escaped such scrutiny.

In January, the gender issue took center stage when Ms Warren and Mr Sanders sparred over whether he had told her in 2018 that a woman could not beat Mr Trump.

While Mr Sanders denied saying so, Ms Warren took the opportunity to remind people of a simple fact.

"The only people on this stage who have won every single election that they've been in are the women, Amy and me," she said during a debate that night, referring to fellow candidate Senator Amy Klobuchar.

In the ensuing weeks, Ms Warren argued more forcefully that women can win, noting female candidates had seen greater success than men when Democrats won the US House of Representatives in the 2018 congressional elections.