(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.
The donation is a boon for the Democratic organization, which had just over $29 million in the bank at the end of August despite raising over $95 million since January 2017, according to the Federal Election Commission.
Democrats' chances of taking back the Senate have improved in recent months, despite the fact that the party is defending 26 seats in November, including two held by independents. While the party finds itself with difficult races in Indiana, Missouri and North Dakota, the possibility that Democrats could unexpectedly win in Tennessee and Texas and flip seats in Nevada and Arizona has opened up the narrow possibility that the party could control the legislative body come 2019.
Republicans currently hold a 51-49 advantage in the Senate.
Bloomberg adviser Kevin Sheekey said in a statement that the former mayor's decision to donate to the group stems directly from his view of Republican leaders like Sen. Mitch McConnell.
"Mike was extraordinary disappointed in the Republican leadership in the Senate and feels increasingly passionate about changing it," Sheekey said. "And he's already enthusiastic about the impact he's having on House races and increasingly confident that he can contribute to a Democratic takeover."
Speaking with CNN in September, Bloomberg said he was undecided on whether to run for President in 2020, but people close to him have said he is carefully eying a run against Trump.
"Right now I'm only focused on the midterms," Bloomberg told CNN's Fareed Zakaria. "I believe that the Republicans have not done what they should have done in terms of providing some counterbalance to the executive branch."
He later said he would think about running after November.
Bloomberg told the New York Times this year that if he were to run for President in 2020 it would be as a Democrat, not as a Republican, and his donations in 2018 are a clear sign that he is trying to deepen his ties to a party whose politics have moved away from left-leaning independents like the former mayor.