Former New York City Mayor Michael Bloomberg (I) has donated $2.5 million to the Senate Majority PAC, Democrats' main outside group aimed at protecting their majority in the upper chamber. 

The hefty donation shows Bloomberg could be a major player in helping Democrats retain Senate control in the 2014 elections.

The billionaire former mayor has indicated he'll stay involved in politics following the end of his 12 years in office earlier this month, and has prioritized helping candidates who support gun control and environmental issues. He's mostly supported pro-business Democrats in recent years.

Bloomberg adviser Howard Wolfson confirmed the donation to The Hill but wouldn't say whether Bloomberg would definitely continue to give to the group, as the election year ticks on. Politico first reported the donation. 

"As to going forward, Mike Bloomberg will support members of both parties who stand up and do the right thing," Wolfson said in an email.

The group, run by former senior staffers to Senate Majority Leader Harry Reid (D-Nev.), was a major player in the 2012 elections. The $2.5 million donation from Bloomberg marks nearly as much as the $3 million the organization raised in the entire first half of 2013.