In 2001 , the incumbent mayor of New York City , Rudy Giuliani , was ineligible for re-election , as the city limited the mayoralty to two consecutive terms . Several well-known New York City politicians aspired to succeed him . Bloomberg , a lifelong member of the Democratic Party , decided to run for mayor as a member of the Republican Party ticket . Voting in the primary began on the morning of September 11 , 2001 . The primary was postponed later that day , due to the September 11 attacks . In the rescheduled primary , Bloomberg defeated Herman Badillo , a former Congressman , to become the Republican nominee . Meanwhile , the Democratic primary did not produce a first-round winner . After a runoff , the Democratic nomination went to New York City Public Advocate Mark J. Green
September 11 attacks
September 11 attacks
Michael Bloomberg