HARRISBURG — Tom Wolf had been heavily favored in polls for almost two months to win the four-way Democratic gubernatorial primary election before an opponent pulled the trigger on a TV attack ad against him.

Five more TV attack ads followed in the last three weeks, including one by the Republican incumbent, Gov. Tom Corbett, as Wolf’s foes made a calculated bet that the risky strategy was the only way to beat him.

They bet too late, it seems.

Wolf is polling so far ahead that a victory by any other Democrat in the May 20 primary election would amount to the most dramatic comeback in Pennsylvania’s modern political history.

“It would be, in my humble judgment, without precedent,” said G. Terry Madonna, a pollster and a professor of public affairs at Franklin and Marshall College in Lancaster.

Christopher Borick, a political-science professor and pollster at Muhlenberg College in Allentown, echoed that sentiment.

“It’s certainly a formidable challenge to reel him in at this point,” Borick said. “It’s just looking at the math.”

So far, three polls in the race have shown Wolf leading by at least 20 percentage points, with the most recent one surveying voters in late April. Overcoming such a large polling deficit is certainly possible, but Madonna and Borick could not recall it ever happening in Pennsylvania in such a short stretch of time.

They acknowledge it is more difficult to produce an accurate poll when voter turnout is low. Without a hot-button issue to motivate the electorate, perhaps 1 million registered Democrats, or one in four in Pennsylvania, will vote in the primary.

Wolf’s advantage started with his spending advantage — the wealthy businessman is helping his campaign outspend his opponents by $5 million.