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(Reuters) - Grassroots donors gave about $1 billion to Democratic candidates and progressive groups in 2019 through the online fundraising organization ActBlue, the nonprofit said on Thursday.

About a third of the money arrived during the last three months of the year, with more than $20 million donated on Dec. 31 alone, the group said.

Small-dollar donations are crucial to Democrats vying for their party’s nomination to run against Republican Donald Trump in November, many of whom have sworn off fundraising from deep-pocketed political action committees in favor of grassroots support. ActBlue, founded in 2004, streamlines the donation process by enabling supporters to give with a click of a mouse or via mobile phones and tablets.

The organization would not say how much of the $1.05 billion raised in 2019 was for presidential campaigns, but executive director Erin Hill said the amount was indicative of support for Democratic causes.

“Based on what we saw last year, the eventual Democratic nominee will have an army of grassroots donors behind them,” Hill said in a press release.

Six million donors used the service to contribute to more than 13,000 campaigns in 2019, the organization said, some making multiple contributions. The average donation was for $30.50. By comparison, during the last presidential cycle in 2016, the organization collected about $207 million for about 3,600 campaigns, with an average donation of $33.13.

ActBlue is not the only organization to funnel online political donations to campaigns. WinRed, which was established last year to raise money for Republican and conservative causes, said on Monday that it had raised $101 million in its first 190 days, about $70 million of it in the final three months of 2019.