Former New York City Mayor Michael Bloomberg said Wednesday that he changed his party affiliation and reregistered as a Democrat.

“At key points in U.S. history, one of the two parties has served as a bulwark against those who threaten our Constitution,” he wrote 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.”

Mr. Bloomberg, formally an independent, has changed parties several times over the past two decades. He shifted closer to his former party as he pledged in June to spend $80 million to support Democrats in the midterms.

The media and business titan will reportedly run for president, potentially setting up a battle of the billionaires in 2020.