(CNN) Vermont Sen. Bernie Sanders raised an eye-popping $6 million during the first 24 hours of his presidential campaign, far surpassing the totals announced by his rivals for the Democratic nomination

But another number he revealed Wednesday could prove crucial to his candidacy and a model for others: the $600,000 he said he's received from people who have signed up to automatically donate to his campaign each month.

In a year's time -- just as Democrats in Nevada and South Carolina weigh in on the presidential nomination -- this initial round of donors already will have contributed $7.2 million to Sanders' presidential ambitions -- with little effort or expense from his campaign.

This "subscription" approach to politics is an increasingly popular way for candidates to build a sustaining fundraising base.

About 20% of the money that flowed to Democratic candidates and organizations through ActBlue in the 2018 cycle came from recurring contributions, Caleb Cade, an ActBlue spokesman, told CNN. ActBlue does not disclose the fundraising details of the individual candidates who use its online platform.

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