Contenders from both parties have been waiting for months to hear what the former mayor would decide.
It was not clear what prompted the decision, but the prospect of potentially facing Attorney General Andrew M. Cuomo, who is quietly planning his own run for governor, may not have appealed to Mr. Giuliani, who suffered a bruising defeat in the 2008 Republican presidential primary. While many political analysts believe Mr. Giuliani would have comfortably beaten Gov. David A. Paterson, he would likely have faced an uphill battle against Mr. Cuomo, one of the most popular politicians in the state.
It remains unclear if the former mayor is considering any other political race in 2010. Some have urged him to take on the newly-installed Senator Kirsten E. Gillibrand, who has never run statewide and is still introducing herself to voters in parts of the state.
Mr. Giuliani has built a lucrative career as a motivational speaker, and his business interests, including his firm Giuliani Partners, tend to suffer when he runs for office, those who know him say. He also has strained relations with Edward F. Cox, the new state Republican Party chairman, which would have complicated a potential campaign. Mr. Cox has publicly urged Mr. Giuliani to consider running for the Senate instead of governor.
Maria Comella, a spokeswoman for Mr. Giuliani, said Wednesday that it was “premature” to say any decisions had been made, and cited the former mayor’s statements on television last weekend.
In an interview with ABC on Sunday, Mr. Giuliani said he had not yet made his decision but said, “I will very soon.”
Those told of the decision spoke on condition of anonymity, because they did not want to speak publicly ahead of Mr. Giuliani.
The decision will be a boost to other Republican contenders. So far, Rick Lazio, a former congressman from Long Island, is the only declared candidate on the Republican side. Another much discussed potential contender is Chris Collins, the Erie County executive.