Washington (CNN) Sen. Kamala Harris' abrupt departure Tuesday from the 2020 presidential race underscores how much the financial landscape has shifted for Democratic presidential contenders -- richly rewarding the candidates who can appeal to small-dollar donors and hobbling some of those competing for the favor of the wealthy contributors who have traditionally fueled campaigns.

The two fundraising leaders among Democrats -- Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts -- each outraised Harris by more than 2-to-1 during the most recent fundraising quarter while relying on small contributions for roughly 60% of their individual contributions.

"My campaign for president simply doesn't have the financial resources we need to continue," Harris said in an email to supporters announcing her departure. "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' departure came just hours after news emerged that a new pro-Harris super PAC had reserved nearly $500,000 worth of television advertising time in Iowa, the state on which Harris had staked her entire campaign. The group quickly canceled the buy Tuesday.

But the pro-Harris People Standing Strong was the third such outside group to crop up in support of Democrats, despite the candidates' early denunciations of big-money super PACs. Super PACs can raise unlimited amounts from rich donors and corporate interests to sway elections.

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