Democratic presidential aspirant Sen. Bernie Sanders attracted a huge crowd Saturday at a rally at the University of Iowa. (Lucian Perkins /For The Washington Post)

Sen. Bernie Sanders's campaign announced Sunday that it will bring in more than $20 million this month, an astonishing sum that underscores the power of its online fundraising operation.

Sanders has received more than 3.25 million contributions, including 770,000 in January, the campaign said. Because most were small sums, more than 99.9 percent of his donors have not yet reached the maximum $2,700 they can give to him in the primary. The New York Times first reported Sanders's January fundraising total.

In all, more than 1.3 million individuals have donated to the campaign, according to spokesman Michael Briggs. Hillary Clinton's campaign has said it has received donations from "hundreds of thousands" of people.

Jeff Weaver, Sanders's campaign manager, said that they are on pace to beat Clinton's goal of raising $50 million in the first quarter of the year.

“Working Americans chipping in a few dollars each month are not only challenging but beating the greatest fundraising machine ever assembled," he said in statement.

Sanders came close to matching Clinton's fundraising haul in the last three months of 2015, raising $33.6 million to her $37 million.

The Sanders campaign also said in its news release that he has "refused to coordinate with a super PAC, which Clinton and other candidates have used to take unlimited contributions from millionaires and billionaires."

However, the senator from Vermont does have a network of independent groups supporting him, including a super PAC financed by National Nurses United, which reported raising $2.2 million in the last half of 2015 and spending $1.3 million. The super PAC is financed by both mandatory and voluntary dues paid by its 185,000 members, officials told The Washington Post.

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Among its expenditures was $245,000 it gave to the liberal super PAC Progressive Kick. Of that, $45,000 was allocated to People for Bernie, a grassroots effort to promote Sanders through social media, The Post previously reported. The nurses' super PAC also gave $150,000 to Reclaim Chicago, an organization mobilizing community leaders to support the senator.

Meanwhile, Priorities USA Action, the main super PAC backing Clinton, announced Friday that it had raised $50.5 million through the end of January and has another $42 million in commitments. A second pro-Clinton super PAC, Correct the Record, reported raising $2 million through the end of December, including a $1 million donation from Priorities USA. Unlike other super PACs, Correct the Record coordinates directly with the Clinton campaign by relying on a 2006 Federal Election Commission regulation that declared that content posted online for free, such as blogs, is off-limits to regulation.