I don’t understand why this guy ever ran as a Republican.
Because it was easier to do so. He only had to use his money to blitz a legitimate Republican, Herman Badillo, in the ‘01 GOP primary. To have run in the Democrat primary would’ve meant navigating the classic minefield of racial group politics, and he didn’t want to do it. The reason the Democrats haven’t won the Mayorship since 1989 (and even at that, by an underwhelming margin) is because there is usually a 3-way squabble between Blacks, Hispanics, and liberal White Jews. When one group outdoes another to get the nomination, they get “even” by endorsing the Republican. When a White Jew, Mark Green, won the 2001 nomination on the Dem side over a Hispanic, Freddy Ferrer, the Hispanics supported Bloomberg (who, ironically, is also a White Jewish liberal - because he wasn’t Mark Green !).