PENNSYLVANIA'S Democrats will select their nominee for governor one week from today. But front-runner Tom Wolf is already being targeted by Gov. Corbett and his Republican allies as the likely general-election challenger.

Corbett is running TV ads critical of Wolf's 18 months as state revenue secretary under Gov. Ed Rendell.

The state Republican Party sent campaign mailers to Democrats, accusing Wolf - without providing proof - of dodging taxes with his family's business.

Pollster G. Terry Madonna of Franklin & Marshall College says the singular Republican focus on a Democrat - before primary election ballots are even cast - is a first for a statewide race here.

The other three Democrats in the race - U.S. Rep. Allyson Schwartz, state Treasurer Rob McCord and former state Environmental Protection Secretary Katie McGinty - yesterday faced one another and Wolf in Philadelphia, attending the last two debates of the primary season.

The candidates have struggled partly because they have few disagreements on major state policy issues, Madonna noted.

They all want an extraction tax on natural-gas drilling in the Marcellus Shale region to help pay for public education. They want to expand Medicaid and increase the state's minimum wage.

Wolf, little-known in statewide politics, jumped out to a double-digit lead in the polls by investing early in television commercials. The other Democrats have been trying to catch him since then.

"This has been one of the most successful introductions of an unknown candidate in modern Pennsylvania history," said Madonna, who will release a new poll on the Democratic field tomorrow.

Wolf's sustained surge prompted changes in how the Republican Party and Corbett hoped to shape the Democratic field. Early GOP tactics suggested they were pushing for a Schwartz victory, considering her an easier candidate to defeat in November.

Now the focus is all on Wolf., a nonpartisan voter-advocacy organization at the University of Pennsylvania, last week picked apart Corbett's anti-Wolf campaign commercial, which claims that "taxes went through the roof" when Wolf was the state's revenue secretary. found that "no broad-based taxes were raised during Wolf's 18 months in office."

The state Republican Party mailer makes similar claims and also accuses Wolf of using the "Delaware loophole" to avoid paying business taxes.

Wolf is chairman and CEO of the Wolf Organization, which supplies kitchen cabinets. He sold a controlling interest in that business in 2006 but repurchased it in 2009 when it was on the brink of financial ruin.

A private-equity firm that still owns part of the company had chartered it in Delaware.

Some Pennsylvania companies set up corporate cousins in Delaware, where state law lets them charge the Pennsylvania side of the business for company trademarks or patents. That can reduce Pennsylvania profits and the taxes that come with them.

Wolf yesterday said his company does not use the Delaware loophole and has no business set up in that state to charge itself for the use of trademarks or patents.

"We're in 28 states," he said of the business. "We're registered to pay taxes in all those states."

Megan Sweeney, a spokeswoman for the state Republican Party, stood by claims that Wolf used the loophole to dodge taxes but offered no proof. Instead, she complained that he has not released his corporate-tax returns.

Wolf yesterday responded with a robocall to Democratic voters, asking them to reject the "smear campaign" from Republicans.

"Republicans are spreading these lies to Democratic voters like you to try to stop me from winning the Democratic primary," Wolf says in the robocall.

Wolf and the Republicans have one thing in common: secrecy in campaign tactics.

The state GOP would not say how many Democrats are getting its fliers; Wolf's camp won't say how widespread his robocalls have been this week.