Story highlights Their haul was surprising

It exceeds the record amount that this cycle's top presidential super PAC

(CNN) It's not just Senate Republicans boasting of huge donations.

The main super PAC boosting Senate Democrats, Senate Majority PAC, will report its most impressive fundraising haul yet Thursday after raising $19.3 million in the first three weeks of October. That sum -- arriving at the door as Senate Democrats grew increasingly likely to recapture the upper body -- is $7 million more than it has raised in any complete month ever, never mind an abridged three-week reporting period.

The group did not immediately release a list of its contributors, which will become public later Thursday -- but the quick fundraising pace strongly hints at a massive gift by one or a few Democratic megadonors.

And despite it being a down-ballot group, their haul surprisingly exceeds the record amount that this cycle's top presidential super PAC, the pro-Hillary Clinton group Priorities USA, raised in October. Backed by Senate Democratic Leader Harry Reid, the Senate Majority PAC said it had $13 million on hand as of October 20.

Two years ago at this same point, the group had just one-quarter of that amount in its war chest.

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