"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", she wrote in a note to supporters. "I'm not a billionaire".
Harris revealed the news early Tuesday and said, "To my supporters, it is with deep regret-but also with deep gratitude-that I am suspending my campaign today".
Harris, 55, entered the race a rising star.
With the December debate coming up, the candidates seem to be dropping like flies.
The California Democrat told her senior staff of the decision Tuesday morning and later sent an email to supporters. Harris had refocused on Iowa, in hopes of achieving a springboard to electoral success.More news: Janet Jackson Surprises Rihanna Onstage At 2019 Fashion Awards
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"I'm moving to Iowa", the senator joked at rallies, as she vowed to campaign in the state each week.
Despite beginning the race as a serious contender for the Oval Office - she regularly polled as a top-tier candidate - her campaign has struggled to maintain its momentum, plummeting in the polls amid reports of turmoil among campaign staff and the lack of a clear strategy.
Harris was once considered a front-runner in the crowded Democratic field but saw her campaign eclipsed by rivals such as Elizabeth Warren and Pete Buttigieg. A couple of her best moments include the most recent debate, when she declared "Donald Trump got punked" by North Korea.