Knox was one of the five Democrats who ran in the 2007 Democratic primary won by Nutter.
Knox said he gave serious consideration to running again but realized that to compete, he would have needed to run negative campaign ads. He said he doesn't have the stomach to do it.
He also said his wife didn't want him to run.
In announcing his decision Tuesday, Knox also said he was endorsing Nutter for mayor.
That currently leaves Nutter with no major challengers for either the May primary or the November general election.
