Former New York City Mayor Michael Bloomberg rejoined the Democratic Party on Wednesday, going back to his political roots weeks before the Nov. 6 congressional elections and amid speculation that he might run for the White House in 2020.

Bloomberg, the billionaire founder and chief executive of global media company Bloomberg LP, said in a post on social media he was re-registering as a Democrat, citing a potential constitutional crisis under Republican President Donald Trump, a fellow New York businessman.

“At key points in U.S. history, one of the two parties has served as a bulwark against those who threaten our Constitution. Two years ago at the Democratic Convention, I warned of those threats,” Bloomberg wrote in post on Instagram.

“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, 76, was a longtime Democrat who became a Republican before running for mayor of the largest U.S. city in 2001. He left the Republican Party to become an independent in 2007 and served three terms as New York’s mayor, stepping down in 2013.