An aide told NBC News that the senator notified her staff Tuesday that she was dropping out and the campaign emailed the news to supporters soon after.
In the email, Harris said her campaign "simply doesn’t have the financial resources we need to continue."
"I'm not a billionaire. I can't fund my own campaign," Harris continued. "And as the campaign has gone on, it's become harder and harder to raise the money we need to compete. In good faith, I can't tell you, my supporters and volunteers, that I have a path forward if I don't believe I do."
She added, "So, to you my supporters, it is with deep regret — but also with deep gratitude — that I am suspending my campaign today."
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Her exit comes just weeks before the deadline to get off the ballot in California, a move that could spare her some embarrassment if she thought she would lose in her home state. She had already qualified for the debate on Dec. 19 — the only candidate of color to have done so at the moment.
A senior aide to Harris told NBC News that the senator made the decision to drop out on Monday after talking with family and top advisers. Harris, over the weekend, conducted a full audit of the campaign's finances and questioned the sustainability of the cash-strapped campaign. Harris' campaign has not been on the airwaves for months and had laid off several dozen staffers last month.
Harris, according to the aide, did not want to continue to ask supporters to fund the campaign because the current financial situation made Harris feel the path forward to success in Iowa and beyond was no longer possible.
Harris, the daughter of an Indian mother and a Jamaican father, launched her campaign to great fanfare on Jan. 21, Martin Luther King Jr. Day. Days later, she held her first rally in front of a crowd of more than 20,000 people in Oakland.
In April, she reported raising $12 million in the first quarter — second only to Sen. Bernie Sanders, I-Vt., who raised $18 million.
Then, at the first Democratic debate in June, she drew notice for attacking former Vice President Joe Biden for his stance on busing and school segregation. After that, her polling numbers shot into the double digits, including registering at 13 percent in the national NBC/WSJ poll.
But her fundraising began lagging over the summer (she reported in July having raised $11.8 million in the second quarter — trailing South Bend, Indiana, Mayor Pete Buttigieg's $24.9 million, Sen. Elizabeth Warren's, D-Mass., $19.1 million, and Sanders' $18.2 million) and was put on the defensive on health care at the second Democratic debate at the end of July.
Following that debate, her polling numbers dropped to the single digits — and never really recovered.
Amid those problems, Harris' campaign reorganized — laying off some staffers in early states to focus its resources and attention on Iowa.
The latest RealClearPolitics average of recent polling showed Harris with just 3.4 percent support nationally, and just 3.3 percent and 2.7 percent backing in the early-voting states of Iowa and New Hampshire, respectively.
Competitors immediately weighed in on Harris' exit.
Biden said he had "mixed emotions" about her exit "because she is really a solid, solid person and loaded with talent."
He didn’t respond to questions about whether he would consider her as a vice presidential candidate if he were to win the presidential nomination.
Sen. Amy Klobuchar, D-Minn., praised Harris in a tweet, calling her a "good friend and incredibly strong public servant."