Kennedy, 51, said that running for the office now held by Eliot Spitzer (search) would have forced him to sacrifice time with his wife and six children. Kennedy said he did plan to run for public office when his children were older. His youngest, a son, is 3-years-old.
"After careful consideration and discussion with my family, I have decided that I cannot take this step at this time," Kennedy, an environmental lawyer and son of slain Sen. Robert F. Kennedy, said in a statement.
He said he wanted to be with his children "to make a difference in their lives while I still can." But he added that a run for public office "will indeed be the next step in my career."
Kennedy is chief attorney for the environmental group Riverkeeper and has been a leading state environmental activist for more than two decades. He had briefly considered running for the Senate seat eventually won by Hillary Rodham Clinton (search) in 2000, and also had cited family commitments when ruling out that bid.
In an interview Tuesday with The Associated Press, Kennedy said his decision had nothing to do with a possible run against Cuomo. Kennedy refused to discuss their relationship.
The very thought of such a race had become instant tabloid fodder.
Cuomo, the son of former Gov. Mario Cuomo, is the estranged husband of Kennedy's sister Kerry Kennedy Cuomo (search). In 2003, when the couple's separation was announced, Cuomo's lawyer created a stir by saying Cuomo "was betrayed and saddened by his wife's conduct during their marriage." The lawyer didn't elaborate.
Last week, Kennedy had said that he didn't want his run against his brother-in-law to overwhelm a discussion of the issues. On Tuesday, however, he told the AP that tabloid coverage was "irrelevant to my decision."
"Clearly, I'd rather talk about the larger issues that I've been working on for 21 years than family issues, but I've lived with the tabloids for 50 years," said Kennedy. "I have a thick skin. And, that's what you expect in New York politics."
Kennedy's decision still leaves a competitive field to replace Spitzer, who is running for governor after using the office to investigate Wall Street, mutual funds and the insurance industry. Besides Cuomo, President Clinton's housing secretary, former New York City mayoral candidate Mark Green (search) and state Assemblyman Michael Gianaris (search) are among the Democratic hopefuls.
Asked if he had a favorite in the race, Kennedy said "a lot of these guys are very talented people." He singled out Green and Gianaris as "terrific." Kennedy did not mention Cuomo.