The California Democrat told her senior staff of the decision Tuesday morning, and later sent an email to supporters.
Harris wrote in the email, "To you my supporters, it is with deep regret -- but also with deep gratitude -- that I am suspending my campaign today."
"But I want to be clear with you: I am still very much in this fight," Harris continued. "And I will keep fighting every day for what this campaign has been about. Justice for The People. All the people."
Harris' exit from the race is a precipitous fall for a candidate who launched her campaign to high expectations. The senator has struggled to move her low poll numbers in recent months and said Tuesday that financial pressures led her to end her bid.
"So here's the truth today. I've taken stock and looked at this from every angle, and over the last few days have come to one of the hardest decisions of my life," Harris wrote. "My campaign for president simply doesn't have the financial resources we need to continue."
"I'm not a billionaire. I can't fund my own campaign. And as the campaign has gone on, it's become harder and harder to raise the money we need to compete," Harris wrote.
Harris had been struggling to gain traction in the race for months. The campaign, in a bid to resuscitate her presidential hopes, turned all of its attention to Iowa in the fall, pledging to keep Harris in the state as much as possible and telling reporters that anything less than a third-place finish in the state would be a loss.
An early sign that Harris' bid was facing challenges came in November when her campaign -- in an effort to further go all in on Iowa -- all but shuttered her organization in New Hampshire and redeployed staff in Nevada and California to Iowa.
The move was an about face for the senator, who had launched her campaign by touting her ability not only to compete in places like Iowa and New Hampshire, but also seize on her local and neighboring status in Nevada and California.
But Harris' departure this early before the Iowa caucuses was unexpected, especially considering she had already qualified for the December debate stage, something a number of candidates still in the race had yet to do.
Harris' departure could have significant ramifications on the candidates still in the race. Her campaign was filled with a number of top Democratic operatives both nationally and in states like Iowa who will be highly sought after by competing operations, and donors that were tied to Harris -- many of whom come from wealthy circles in California -- may now feel more free to back other candidates.
In the email to supporters, Harris said though she is no longer running for president, "I will do everything in my power to defeat Donald Trump and fight for the future of our country and the best of who we are."
The high point of Harris' campaign came during the first Democratic debate in June, when she confronted former Vice President Joe Biden on his past opposition to busing as a way to desegregate schools. The moment went viral, catapulting her in the polls and exposing a weakness in the vice president.
But the days after the moment, it was clear the confrontation did more harm than good, with Harris being accused of manufacturing a moment where there was little policy difference between the vice president and her.
Since that moment, Harris was on the decline, unable to sustain the momentum she had coming into the race.
This story is breaking and will be updated.
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