It notes that last month he said in a New York Times interview that he believes a third-party candidate can not win the presidency, and that were he to run, he would do so as a Democrat. He served three terms as New York City mayor and has variously been a Democrat, a Republican and an independent.
He said in an Instagram post on Wednesday morning he has now registered as a Democrat.
The 76-year-old billionaire has switched political parties several times in the last couple decades. "That's not to say I'm with the Democratic Party on everything, but I don't see how you could possibly run as a Republican".
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Mr. Bloomberg, formally an independent, has changed parties several times over the past two decades. Two years ago at the Democratic Convention, I warned of those threats, ' Bloomberg wrote.
Bloomberg, 76, was a longtime Democrat who became a Republican before running for mayor of the largest US city in 2001.
"It's impossible to conceive that I could run as a Republican - things like choice, so numerous issues, I'm just way away from where the Republican Party is today", he told the Times.
The idea now, of course, is that Bloomberg will run for president in 2020, and that he'll do so as a Democrat. Last week, he announced he was putting $20 million behind Democratic efforts to flip the Senate.
Bloomberg, who in the past has donated to both Democratic and Republican candidates, has said he will spend US$80 million (RM332.3 million) on Democrats alone ahead of November's key polls.