Washington (CNN) Michael Bloomberg, the billionaire former New York City mayor and potential 2020 presidential candidate, announced Wednesday that he re-registered as a Democrat, saying the party must "provide the checks and balances our nation needs so badly."

"At key points in U.S. history, one of the two parties has served as a bulwark against those who threaten our Constitution," Bloomberg wrote in an early morning Instagram post.

"Two years ago at the Democratic Convention, I warned of those threats. 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."

A Bloomberg spokesman declined any further comment.

Bloomberg, who was elected mayor as a Republican and an Independent, has been toying with the idea of running for president in 2020 as a Democrat. In September, he told The New York Times that if he were to run in two years, it would only be on the Democratic ticket.

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