(CNN) Former New York city mayor and billionaire Michael Bloomberg is putting $20 million behind Democratic efforts to flip the Senate in the 2018 midterm elections.

The donation will go to the Senate Majority PAC, the main vehicle for the Democratic Party's push to retake the majority in the Senate.

Bloomberg was already slated to be one of the largest Democratic donors of the cycle after he pledged to spend $80 million on Democratic congressional candidates earlier this year. But by adding $20 million to his planned spending -- bringing the expected spending to around $100 million -- Bloomberg is firmly positioning himself with Democrats as he contemplates challenging Trump in 2020.

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

The Washington Post first reported the move.

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