On its face, it makes perfect sense. Bloomberg was never a Republican in any true sense of the word. When he first ran for office in 2001, he decided to do so as a Republican because the Democratic primary was already crowded with well-known candidates. The Republican nomination was his for the taking, and he took it. Then, in the general election he used his vast personal wealth and his pitch to bring a businessman's sensibility to the job to overcome the city's strong Democratic leanings. Now that he has been elected to two terms, Bloomberg has no need to remain in a party that he disagrees with on any number of issues.
So basically he's always been a Democrat but just changed the labels. I figured as much as I don't see a distinction beteen Mayor Koch and Bloomberg or Mario Cuomo or his son.
He’s really a member of the Opportunist Party.