PHILADELPHIA - Front-runner Tom Wolf defended his record Thursday in a debate that plowed little new ground in the final days of the four-way race for the Democratic nomination for governor.

In the opening question of the hour-long discussion at WPVI-TV, fellow candidate Rob McCord was asked whether he would take down a TV attack ad that questions Wolf's relationship with a former York mayor charged with murder and acquitted in the death of a black woman during the city's 1969 race riots.

McCord, Pennsylvania's state treasurer, said he did not intend to stop airing the ad.

"I felt compelled to put it on the air as a matter, actually, of principle," he said. "We have to know how to confront issues of racism."

Wolf said he agreed to head then-Mayor Charlie Robertson's re-election campaign in 2001 because of his accomplishments in office. But after Robertson was charged, Wolf said he helped persuade him to abandon his campaign.

"I was instrumental in getting Mayor Robertson to withdraw from the race," Wolf said "And I thought the way I did it was appropriate."

Thursday's debate was taped and is scheduled to be aired this weekend. A final debate is Monday night at Drexel University in Philadelphia. The primary election is May 20, and the candidates are competing for the Democratic Party nomination to challenge Republican Gov. Tom Corbett in the Nov. 4 general election.

On another issue that has dogged Wolf for weeks, U.S. Rep. Allyson Schwartz charged that Wolf was "leaving out part of the story" in describing the 2006 sale of his family-owned building products company and his rescue several years later of the company, the York-based Wolf Organization. She said Wolf had profited from the sale, while hundreds of employees were being laid off.

Wolf said the layoffs occurred after his departure, as the company shrank during the national recession.

"I did go back and save that business" in 2009, as well as the jobs it provides, he said.

The rescue that Wolf portrays is part of a campaign narrative, which he has told to voters through millions of dollars in TV ads, that also includes stints as a Peace Corps volunteer in India and as state revenue secretary.

"I know it's a tough story to believe," he said. "It is a very compelling story and it's true."

The candidates reiterated their shared positions on numerous issues, including their support for legalizing same-sex marriage in Pennsylvania, taxing the natural-gas drilling industry and restoring the approximately $1 billion that Republican Gov. Tom Corbett made in budget-balancing cuts to education aid during his first year in office.

Katie McGinty, a former state environmental protection chief, summed up the candidates' united opposition to a law that would have required most Pennsylvania voters to show photo identification at the polls.

McGinty called it "nothing short of a disgrace" that Corbett spent taxpayers' money unsuccessfully defending the statute in court.