Pennsylvania's top two Democrats called Saturday on state Treasurer Rob McCord to pull the racially charged attack ad he is airing against front-runner Tom Wolf in the gubernatorial primary.

Former Gov. Ed Rendell said the television spot was "one of the worst I have ever seen," adding that it was a style of politics "that makes me ashamed to have been part of this business for most of my adult life."

McCord, in an emotional news conference, later said he would continue to use the ad, calling it a "teachable moment" about the moral need to confront racism.

Sen. Bob Casey called the ad "offensive" and said it slanders Wolf's character by implying he is insensitive to racism. "Tom is a man of uncommon integrity, with an abiding sense of fairness and justice," Casey said in an interview.

The 30-second spot questions why Wolf served as chairman of the 2001 reelection campaign of York Mayor Charlie Robertson, who was indicted the day after the primary on murder charges in the death of a black woman during the city's 1969 race riots, when he was a police officer.

Robertson acknowledged holding bigoted views at the time, but said he had changed by 2001, and he denied involvement in the killing. He was acquitted.

In recent weeks opponents have been peppering Wolf with attacks on his record as a business owner and his personal finances, attempting to chip away at what the latest public poll last week found to be a 25-point lead over McCord and Rep. Allyson Y. Schwartz. Former state environmental secretary Katie McGinty, who has not joined in the attacks, is farther behind.

The campaign turned more personal and negative Friday when McCord launched an ad that puts a harsh spin on the Robertson episode.

"Why would he chair the campaign of a man arrested for his role in a race riot, one that left a black woman dead?" the ad says. "For York, it was an ugly episode; for Tom Wolf, there's just no good answer."

The ad was made by consultant John DelCecato, a partner in AKPD Message & Media, who has worked on President Obama's campaigns and was media strategist for New York Mayor Bill de Blasio.

The Wolf campaign fired back with an ad featuring the current York mayor, Kim Bracey, who is African American. She calls Wolf her mentor and says that McCord should be "ashamed of himself."

Wolf, a prominent business leader in York, had worked with Robertson on redevelopment issues.

After the mayor was arrested, Wolf was quoted in a local newspaper offering to stay on for the general election campaign. In any event, Robertson dropped out of the race within a week.

At the time, current state Auditor General Eugene DePasquale was chairman of York County's Democratic Party. He recalled in an interview Saturday that he, Wolf, and Charlie Baca, a close adviser to Robertson, conferred by phone and agreed that the mayor could not continue as the party's nominee.

Because Wolf and Baca knew Robertson best, "it fell to them to give the mayor the news, so to speak," DePasquale said. "It was absurd to think the guy could run, to put the community through that."

DePasquale said he was not in the meetings among the two men and Robertson, so he does not know what was said. "Several days later, Tom called me and said the party was going to get a faxed letter in a half-hour from the mayor announcing his withdrawal," DePasquale said.

McCord, his voice sometimes breaking as his wife, Leigh Jackson, stood beside him in the AFSCME Local 1199 union hall in Center City on Saturday, said that the issue of racism was personal to him. He said that Wolf "had his gut-check moment 13 years ago" and failed it.

"Even if I am in a minority, I'm going to continue the conversation," McCord said. "Leigh and I have been together more than a quarter century and we've seen some things we wish we hadn't." Jackson is African American.

McCord said that he disagreed with Rendell and would not pull the ad.

"If I lose the support of more people instead of gaining it, I would be proud to have pushed this conversation forward," McCord said.

Rendell, who called a news conference Saturday to address the ad, said he understood the pressure McCord was under.

"I know the feeling of desperation when you're losing, because you believe in yourself," Rendell said. "And I believe that Rob McCord believes in himself and believes that as governor he could do good things for the people of Pennsylvania. But that desire, that knowledge, should not overcome basic decency."

Neither Rendell nor Casey has endorsed anyone in the governor's race, and they made it clear they were not doing so now. They said they were concerned the increasingly vituperative tone of the campaign could harm the Democrats for the fall race against Gov. Corbett.

"We're going to have to unify and come together as a party to defeat a well-financed incumbent," Casey said. "We don't need a campaign that makes that harder." He said that the candidates' policy views and records in office or business were fair game, but not character attacks.