Sen. Elizabeth Warren announced Thursday that she is suspending her presidential campaign, a bitter blow for a senator who was long seen by prominent Democrats as headed for the White House.

Warren said she would not be making an endorsement of another candidate right away.

"I will not be running for president in 2020, but I guarantee I will stay in the fight," Warren told reporters and supporters outside her Cambridge, Mass., home.

In emotional remarks, Warren reflected on the role that sexism might have played in the campaign, saying, "One of the hardest parts of this is all those big promises and all those little girls who are going to have to wait four more years."

The Massachusetts Democrat initially announced her decision to leave the race on a call with staff Thursday morning in which she expressed disappointment, but thanked them "from the bottom of my heart" for what they were able to accomplish.

"What we have done — and the ideas we have launched into the world, the way we have fought this fight, the relationships we have built — will carry through, carry through for the rest of this election, and the one after that, and the one after that," Warren said, according to a transcript of the call provided by her campaign.

The campaign proved that grassroots organizing and fundraising are possible in a presidential race and brought several substantive policy proposals to the fore, including a wealth tax, universal child care and canceling student loan debt, Warren said.

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"I may not be in the race for president in 2020, but this fight — our fight — is not over," Warren said. "And our place in this fight has not ended."'

Sen. Bernie Sanders, I-Vt., told reporters on Thursday that he "had a positive" talk with Warren and said there was "no question" that Warren's platform is more ideologically similar to his policies than former Vice President Joe Biden's campaign.

He asked her supporters to come into his camp as he tries to preempt Biden for the Democratic nomination.

“I have known Senator Warren for some 20 years, and she is very focused, very disciplined, very hard-working," Sanders said. "And so, today, I would simply say to her supporters out there, of whom there are millions, we are opening the door to you, and we'd love you to come on board together. Then we can win this primary process together we can defeat the most dangerous president in the history of American politics."

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Sanders wrote earlier Thursday in a post on Twitter that Warren had run "an extraordinary campaign" and without her, "the progressive movement would not be nearly as strong as it is today."

Sen. Warren has run an extraordinary campaign of ideas – demanding that the wealthy pay their fair share, ending corruption in Washington, guaranteeing health care for all, addressing climate change, tackling the student debt crisis and vigorously protecting women's rights. — Bernie Sanders (@BernieSanders) March 5, 2020

In 2016, Warren declined to endorse either Hillary Clinton or Sanders during their heated primary battle, disappointing supporters of both candidates, and instead sought to use her leverage to shape the eventual nominee’s agenda.

Allies say Warren’s ultimate goal is to enact her progressive priorities, and that it is in her nature to think about how to maximize her leverage.

Clinton has been in touch with both Warren and Minnesota Sen. Amy Klobuchar, who also dropped out of the presidential race this week, a source familiar with the outreach said.

Warren's decision to leave the race, which comes after a poor Super Tuesday showing in which she failed to win any states, including her home state, ends a frantic year of campaigning for a candidate who branded herself as a progressive fighter from humble beginnings who was ready to take on a broken and corrupt system.

Warren wowed crowds with her sharp intellect, her clear prognoses for complex problems and her endless stream of policy blueprints to tackle them. After a long polling rise over summer that continued into the fall, it was clear Warren's message was catching on, and she rose to the front of the pack in some polls while avoiding conflict with rivals. In late summer, she drew massive crowds of about 15,000 in Seattle and 20,000 in Washington Square Park in New York.

The plan was working. Then it all started to fall apart.

Warren was pummeled on her "Medicare for All" plan by rivals like Pete Buttigieg, who started to peel away white college graduates, a group that formed the core of her base. She responded by releasing a series of detailed financing mechanisms, followed by a plan to transition onto Medicare for All in the third year of her presidency, moving first to establish a “public option” in the first year. The move didn't placate moderates and sowed doubts about her among left-leaning voters, who instead moved toward Sanders.

"The problem that Warren has is all of the Bernie people think she's a neoliberal shill, and all of the centrists think she's a raging Maoist," Sean McElwee, a progressive organizer and co-founder of Data For Progress, told NBC News last month.

Meanwhile the crowded field of moderates, most notably the staying power of Biden, made it harder for her to grow her support among the mainstream Democrats who backed Hillary Clinton in 2016.

Some Warren supporters believe Biden’s surge following South Carolina cost her dearly on Super Tuesday.

“Joe Biden’s three-day news blitz replaced an entire year of campaigning and organizing,” said Adam Green of the Progressive Change Campaign Committee, which supported Warren. “It’s a little frustrating.”

Questions about "electability" also dogged Warren throughout the fall as many voters, haunted by the last presidential election, fretted that the country wouldn't elect a woman.

It was a quiet, even subconscious fear, thrust into the spotlight right before the Iowa caucuses when news broke of a late 2018 meeting between Warren and Sanders, during which Warren said Sanders told her he didn’t think a woman could beat President Donald Trump.

The story led to a January debate stage confrontation, with Sanders denying he said it and Warren sticking by her re-telling of the story. The two later tried to quell the tensions — not wanting to alienate their progressive supporters. But the moment also made it harder for Warren to draw contrasts with her fellow progressive, at a time when it would’ve benefited her to show how she was different.

She wasn’t helped by polls that showed her faring worse than Sanders and Biden against Trump in key states like Michigan and Florida, reinforcing doubts among Democrats whose top concern was the only constant in this primary: finding a candidate who they could trust to defeat Trump.

Trump nourished this fear by repeatedly attacking her as “Pocahontas,” a reference to her decades-long claim of Native American ancestry and her 2018 decision to take a DNA test to prove it.

The president lost no time remarking on her withdrawal from the race, claiming she was days late in dropping out, costing Sanders wins in some Super Tuesday states.

Elizabeth “Pocahontas” Warren, who was going nowhere except into Mini Mike’s head, just dropped out of the Democrat Primary...THREE DAYS TOO LATE. She cost Crazy Bernie, at least, Massachusetts, Minnesota and Texas. Probably cost him the nomination! Came in third in Mass. — Donald J. Trump (@realDonaldTrump) March 5, 2020

Ultimately, it was perhaps the crowded field that hurt Warren most. National and early state surveys of Democrats heading into the new year showed her leading all rivals on "second choice" preferences. Her campaign sought to turn that into a positive by debuting a new message of unity, pitching her as the candidate best-positioned to bridge the divides between the party's left and moderate wings.

That, too, backfired. For many voters it was difficult to square “unity” with her “fighter” persona. Previous candidates attempting to bridge that divide had also failed, including Sens. Kamala Harris and Cory Booker.

After finishing just fourth and third in Iowa and New Hampshire, respectively, Warren dispensed with that message and revived her more combative persona, launching blistering attacks on new arrival Mike Bloomberg in the Las Vegas debate. She also finally abandoned her nonaggression pact with Sanders. It was a change-up that allies had pushed for for weeks, one that reminded progressives why they were attracted to her in the first place and led to a massive windfall of much-needed donations that kept her campaign afloat.

She followed that up in the Charleston, South Carolina, debate the following week, arguing that she'd be more "effective" at passing a progressive agenda because, unlike Sanders, she sweats the details of policy and process. Despite finishing fifth there, her campaign vowed to keep fighting for delegates and take the fight to the July convention.

But Sanders was already in a dominant position by then, and Biden racked up valuable endorsements after South Carolina that propelled him to big victories on Super Tuesday. After a disappointing day for Warren, in which she won only a small portion of delegates and lost her home state, her campaign signaled that she was reassessing her path forward.

Her departure from the race means there are no more women in the top tier of the field.