THE TOPIC of discussion in a voter forum yesterday with the four Democratic candidates for governor was to be education - and only education.

But York County businessman Tom Wolf has opened up a double-digit lead on his opponents.

And the May 20 primary election is three short weeks away.

So politics swiftly overtook and temporarily displaced policy.

State Treasurer Rob McCord used his opening statement to criticize Wolf for serving in 2001 as campaign chairman for then-York Mayor Charles Robertson, who won the primary election but withdrew from the general election after he was charged with murder for the 1969 death of a black woman during a race riot.

Two men were convicted in 2002 for that killing while Robertson was acquitted.

"I believe that's fundamentally a failure of judgment to recognize the danger of racism and check it," McCord said, calling on Wolf to explain his actions.

The crowd cheered loudly when Daily News columnist John Baer, moderating the forum, reminded McCord that education was the topic of the evening.

McCord kept up the attack, prompting Wolf to note that York's first African-American mayor now holds office and is supporting his campaign.

"I'm proud of what I've done in the community to help York recover from those terrible times," Wolf told McCord.

U.S. Rep. Allyson Schwartz also used her opening statement to ask Wolf for a "better answer" about his support of Robertson.

Schwartz then went after Wolf for his support of former state Rep. Stephen Stetler, a York County Democrat convicted in 2012 on corruption charges.

Only former state Environmental Secretary Katie McGinty joined Wolf in sticking to the topic of education at the forum.

An April 3 Daily News/Franklin & Marshall College poll of registered Democratic voters showed Wolf well ahead with 33 percent. Schwartz trailed at 7 percent while McCord had 6 percent and McGinty posted 4 percent.

That poll also showed that two out of three Democratic voters in Pennsylvania think the state is "headed in the wrong direction."

One out of three of those voters listed education as "the most important problem facing Pennsylvania today," making that the largest issue of concern in the state.

Yet there were few substantive differences among the four candidates on education issues during yesterday's forum.

Each leveled the most common Democratic criticism about Gov. Corbett on education, accusing him of slashing $1 billion in school funding from the state budget.

Corbett insists the $1 billion was onetime federal stimulus money that expired.

About two-thirds of that federal cash was invested in the education budget by former Gov. Ed Rendell at the end of his second term. Corbett used the other one-third at the start of his first term.

Wolf said Corbett's claim of investing more money in education "doesn't ring true" to voters.

"No one forced him to run for governor of Pennsylvania," Wolf said of Corbett. "No one said it would be easy."

The candidates agree a severance tax on drillers tapping the natural-gas deposits in the state's Marcellus Shale region would help boost education funding.

Wolf, Schwartz and McGinty would set that tax rate at 5 percent, similar to other states with natural-gas drilling taxes.

Wolf estimated such a tax "would produce $600 million to $700 million in year one."

McCord is pushing for a 10 percent tax, noting that some Republicans in the General Assembly now support a severance tax but at a lower rate than he favors.

"They're starting to coalesce around too low a number," McCord said.

McCord ended the forum where he started, saying he was fine making the audience "uncomfortable" with questions about Wolf, race riots and Robertson.

Wolf's campaign responded with a news release before the forum ended, denouncing "McCord's and Schwartz's outrageous, racially charged attacks."