Breaking with the state Republican party, Mr Giuliani accused the conservative Mr Pataki of not having a mind of his own and of spouting slogans 'out of a political consultant's playbook'. A little known state senator, Mr Pataki is widely seen as the creation of Senator Alfonse D'Amato, who is the mayor's great political rival in the local Republican party.
The mayor's treachery is especially bold because Mr Cuomo, 62, is a national symbol of the liberal Democratic tradition that is under fire from younger, thrusting conservative Republicans such as 49-year-old Mr Pataki. Mr Cuomo was thrilled, and even President Bill Clinton called Mr Giuliani to thank him.
In making his decision, the mayor judged that Mr Cuomo was clearly 'his own man', even after 12 years in office, and that the governor offered the best hope for change - and the best hope for New York City.
Mr Cuomo is one of the nation's great speakers. But he has not made his mark as governor other than to raise taxes and build more prisons than all his predecessors combined.
The tight race had helped Mr Cuomo widen his perspective, said the mayor.
'Cuomo has learned a great deal from this election and that new insight will give him the strength and determination to establish his place in history,' Mr Giuliani added. The endorsement could prove crucial to Mr Cuomo's chances, as he cannot win without a large turn-out in overwhelmingly Democratic New York City. Mr Giuliani was elected a year ago by white Democrats, who felt he would be tough on crime.
In crossing party lines, Mr Giuliani follows one of New York's great independent- minded mayors, Fiorello La Guardia, the Republican maverick who led the city from 1933 to 1945. Mayor La Guardia is Mr Giuliani's role model. Mr Giuliani ran for office on an all-party 'fusion ticket', a proven formula for an ethnically diverse city.