Former New York Mayor Mike Bloomberg announced Wednesday that he re-registered as a Democrat, a move that could foreshadow a possible challenge against President Trump in 2020.

“Today, I have re-registered as a Democrat – I had been a member for most of my life – because we need Democrats to provide the checks and balance our nation so badly needs,” Bloomberg said.

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Bloomberg was a life-long Democrat until 2001, when he ran for Mayor in New York City on the Republican ticket and served three four-year terms.

The multi-billionaire has been registered as an independent since 2007, around the time of the financial crisis, but said he decided to re-register as a Democrat because he believes Democrats need to provide checks and balances to the Republican majority.

“At key points in U.S. history, one of the two parties has served as a bulwark against those who threaten our Constitution,” Bloomberg posted to Instagram, referring in this instance to the Republican Party. “Two years ago at the Democratic Convention, I warned of those threats.”

He also posted the news to his Twitter and Facebook pages.

Bloomberg has spent almost $100 million on House Democratic races for the 2018 midterms, and made a $20 million contribution to Senate Democrats, according to Axios.