Democratic gubernatorial nominee Tom Wolf's income-tax proposal is sounding about as convincing as a low-calorie label on a cheesesteak. The businessman has said he would cut income taxes for poor and middle-income Pennsylvanians while raising taxes on high earners. But in his first debate with Gov. Corbett this week, Wolf said he didn't have "enough data" to determine which incomes would be subject to higher taxes.

Wolf previously said the state Revenue Department's available records are too outdated to specify winners and losers under his plan. Nor has he explained how he would keep roughly $12 billion a year in income-tax revenue flowing into the state's coffers.

Wolf has said he supports a "universal exemption" for all income beneath a certain threshold, along with higher taxes for incomes above that level. That would make the state's constitutionally codified flat tax effectively more progressive by subjecting higher incomes to higher taxes.

But that's about the extent of the detail Wolf has offered, raising questions about who would pay more and how much. That's a shame, because the state's flat income tax places an unfair burden on lower-income taxpayers that ought to be addressed.

It should be noted that Wolf holds a doctorate from the Massachusetts Institute of Technology and used to be the state's revenue secretary. Certainly he has the credentials to make something of the available figures.

If he wants to be governor, Wolf should provide some numbers and explain how his plan would work. And if the calculations have to change based on shifts in the state's economy or finances, he should explain that.

Beyond the murky figures, Wolf's plan faces further uncertainty on legal grounds. The candidate says it won't require amending the state constitution, which requires uniform taxation. Still, his plan would need approval from a legislature likely to remain under Republican control, and it would have to survive inevitable court challenges.

But campaigns are about a vision of what is possible, and Wolf should be able to make the case for a progressive income tax even in the face of these obstacles. His refusal to explain the details of his plan may be calculated to preserve political advantage, but it's helping the Corbett campaign question whether Wolf would impose additional burdens on the middle class. That doesn't seem to be Wolf's intention, but he can't defend his plan with complaints about insufficient data.

Wolf is losing on an issue that could be a win for the state and most of its taxpayers. He should give voters something to sink their teeth into and put some meat on his proposal.