The announcement came as the billionaire media mogul has emerged as a top financial backer of the Democratic Party during the pivotal 2018 midterm congressional races, pledging almost $100 million to Democratic candidates and causes.
He said: "I don't see how you could possibly run as a Republican".
Last week Bloomberg announced that he will donate $20 million to the Senate Majority PAC to support Democratic candidates for the US Senate. He weighed running for the presidency in 2016 but ultimately endorsed Hillary Clinton in a high-profile speech at the Democratic National Convention.
Bloomberg was previously a member of the Democratic party but switched in 2001 to run for NY mayor as a Republican. "So if you ran, yeah, you'd have to run as a Democrat". He apparently changed sides, however, in 2001 to run for New York City mayor as a Republican after Rudolph Giuliani came up against the term limit.
FILE - Hundreds of demonstrators march across the Brooklyn Bridge to call for tougher gun control laws, June 14, 2014, in NY.
A billionaire businessman, Bloomberg has switched political affiliations before.