Former New York City Mayor Michael Bloomberg (I) is continuing his massive spending on behalf of Democrats this election, with plans to give $20 million to Senate Democrats’ main super PAC.

Bloomberg’s big contribution to the Senate Majority PAC (SMP) will provide a serious boost to Senate Democrats in their uphill battle to retake the upper chamber.

“Mayor Bloomberg is a critical ally in our quest to win Senate races in every corner of the country,” Senate Majority PAC head JB Poersch said. “Mayor Bloomberg’s contribution is another indicator that Democrats have real opportunity to take back the Senate.”

The donation nearly doubles the amount of cash the group had heading into the home stretch of the campaign — SMP had $29 million in the bank at the end of August — and could help Democrats stay at parity with Republicans in outside spending in key Senate races in places like Missouri, Indiana and Tennessee.

According to the Washington Post, which first reported the donation, this big bet is in addition to the $80 million Bloomberg had already committed to spend on the midterms. The political independent and former Republican has turned into Democrats’ biggest donor this cycle by far, helping to fund numerous groups aiming to turn Congress blue and act as a check on President Trump.