Bernie Sanders raised more than $1.2 million in less than 48 hours off a pro-Hillary Clinton super PAC’s attack on the Vermont senator, his presidential campaign said Thursday.

That’s an unprecedented response in such a short amount of time, according ActBlue, the non-profit that processes online donations for many Democratic campaigns, including Sanders.

“We’ve never seen an immediate donor response like what the Sanders’ campaign received on Tuesday,” said ActBlue Executive Director Erin Hill. “At one point, it drove 180 contributions through our platform per minute. Over its 11-year history ActBlue has sent money to over 11,000 campaigns and committees – and the Bernie Sanders campaign holds the record for the two biggest donor days ever for a campaign on our platform.”

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The attack came from Correct the Record, which, unlike most super PACs, coordinates directly with Clinton’s campaign. The group has been sending trackers to Sanders campaign events and plying reporters with so-called opposition research about the Vermonter for months, but never with the intention of its work being made public.

But after The Huffington Post reported on a particularly controversial line of attack from Correct the Record earlier this week, the Sanders campaign took offense and saw an opportunity. They sent not one but two fundraising emails to supporters in response, and were astonished by the response.

“It was the kind of onslaught I expected to see from the Koch Brothers or Sheldon Adelson, and it’s the second time a billionaire Super PAC has tried to stop the momentum of the political revolution we’re building together,” Sanders said in one email.

The exchange of fire between forces loyal to the leading Democratic candidates comes as both are dropping the awkward congeniality of the earlier months of the campaign as they head of the first Democratic debate next month. And the haul underscores the difficulty Clinton will have in attacking Sanders, who can turn any attacks around on Clinton as evidence she is nervous and as an attempt by the establishment to silence them.

Correction: An earlier version of this story stated ActBlue is a company. It is a non-profit organization.