Steele's ad, released Tuesday, attacked Castor for not arresting Cosby in 2005, when Temple University employee Andrea Constand alleged that Cosby drugged and molested her in his Cheltenham mansion.
Castor, who was district attorney then and said the case was too weak to charge Cosby, said Thursday that new information became available only after he left office in 2008 to become a county commissioner.
"Kevin Steele could have done something, because he is still a prosecutor who chose to do nothing at all," Castor says in his TV ad.
Dozens of women have come forward in the last year, alleging that Cosby drugged and assaulted them. Nearly all of the cases are outside the statute of limitations to charge Cosby with a crime.
Steele, who is first assistant district attorney, said Castor knew of other victims a decade ago.
"In his own press release in 2005, he laid out the fact that he had 'reviewed statements from other persons claiming that Mr. Cosby behaved inappropriately with them on prior occasions,' " Steele said Thursday, quoting from Castor's 2005 announcement that Cosby would not be charged.
Castor said he remembered statements from one or two women and determined their statements would be inadmissible as evidence to prove that Constand's allegations were part of a pattern of behavior by Cosby.
Thirteen women joined Constand's federal lawsuit against Cosby, filed after Castor declined to file charges, and Constand's attorney has said many of those women were also in contact with the District Attorney's Office.
"This is a case where I think the public has to look at the judgment of Castor and his decision-making, and needs to look closely at how he has talked about victims," Steele said.
In interviews, Castor has said he got justice for Constand by not arresting Cosby because the decision forced Cosby to testify in a civil deposition for Constand's lawsuit, which was settled out of court.
"You see, if you're Andrea Constand, on the one hand you have the possibility of making millions of dollars in a lawsuit," he said on Thursday.
District Attorney Risa Vetri Ferman has neither confirmed nor denied that her office is reconsidering the case.
But Ferman, who is running as a Republican for county judge, has said prosecutors have a responsibility to review past decisions when new information becomes available.
Legal experts have said a revived investigation would depend on Cosby's statements in his deposition for Constand's lawsuit. The transcript was released for the first time this summer.
Though Cosby and his attorneys have repeatedly denied that he assaulted any women, he has recently contacted prominent Philadelphia-area defense lawyers to gauge their willingness to defend him in a criminal case. Dolores Troiani, Constand's attorney in her civil lawsuit against Cosby, has said her client would be willing to participate in a reopened investigation.
Castor called Steele's attack "despicable desperation politics" and said he did not plan to run a negative campaign until he saw Steele's ad.
"That means he must have concluded that there is enough evidence to arrest Cosby," Castor said. "So why hasn't he done it?"
Steele said the decision to file charges lies with the district attorney. He is second in command.
Voters go to the polls Nov. 3 to elect a district attorney. The winner will take office in January, the same month the 12-year statute of limitations on felony sex crimes will run out in the Montgomery County allegations against Cosby.