"In order to win this campaign, it would require me to conduct a negative campaign against Mayor Nutter," said Knox. "That's not who I am."
Knox, who spent $12 million in his 2007 mayoral bid, also said his wife had not been keen on another run.
A Franklin & Marshall College/Daily News Poll released two weeks ago showed Knox trailing Nutter in a general-election rematch 46-28 percent with 26 percent undecided. The poll also showed that 53 percent thought that Nutter should not be re-elected to a second term.
Nutter thanked Knox for his support. He also announced that Knox would head up a task force to review the use of city-owned buildings and decide if any should be sold. Nutter said the position had not been provided to hasten Knox's exit.
Rendell said that Nutter had performed well under difficult circumstances and that the power of incumbency makes it difficult to challenge a sitting mayor in Philadelphia.
"When you run against an incumbent, you have to run an almost entirely negative campaign," Rendell said. "You have to say: 'This is wrong, this is wrong, this is wrong.' It has to be tearing down. I've known Tom Knox a long time. He's not negative. He's not interested in tearing down."
So far, former state legislator T. Milton Street, who was released from federal prison last year after serving 26 months for not paying income taxes, is the only Democrat running against Nutter.
John Featherman, a Republican at odds with his own party in Philadelphia, is the only GOP candidate for mayor so far. Ex-GOP mayoral candidate Sam Katz and state Sen. Anthony Hardy Williams both considered primary challenges against Nutter this year but decided not to run. City Councilman Bill Green flirted with the idea but is now expected to seek a second Council term.