Democrat Jason Carter is honing his answer on the question that's dogged him perhaps the most in his race for governor: How he would boost education funding if he defeats Gov. Nathan Deal in November.

He said earlier this month that a comprehensive review of state spending would yield a "giant amount of waste" that could be better spent in classrooms. He added a new wrinkle to that Wednesday by pledging to go after tax cheats to help rev up state revenue.

"There's $2.5 billion that is uncollected out there from people who are cheating on their taxes, and against whom we aren't enforcing the law," the Atlanta state senator said. "That's $2.5 billion dollars of money that's gone uncollected by our state government for years now."

He's referring to a state audit released in January that encouraged revenue officials to step up efforts to collect back taxes. The report estimates that $4.4 billion is owed to the state, but at least $1.9 billion in taxes is unlikely to be collected due to errors, bankruptcies or closed accounts.

That leaves a pot of $2.5 billion that could be tapped. But even that number is squishy. Revenue officials estimate that about $800 million of that sum is from accounts that are at least 15 years old - and thus much harder to collect. By contrast, some $500 million is lower-hanging fruit - from debt less than four years old.

Carter's idea isn't novel. Georgia governors have long vowed to step up tax enforcement by bolstering the revenue department's investigative wing. The audit noted that a $27 million expansion of the tax compliance division partly under Deal's watch (in fiscal years 2011 and 2012) helped hire an additional 150 employees. In all, it said, a $67 million investment on tax compliance activities in 2013 yielded more than $450 million in financial benefits to the state.

Carter said state government should step up its investment in pursuing tax cheats and rooting out wasteful spending.

"If Gov. Deal wants to claim that there is no waste in that state government that can be dealt with, I would happily engage in that discussion," said Carter. "I guarantee you that it's there."

Deal's campaign said Thursday that Carter's latest proposal was a sign he didn't have a "serious" plan to fund education. Deal spokeswoman Jen Talaber called on the Democrat to outline specifically what he would cut to increase schools funding.

"Every government is always fighting and going to court to collect owed taxes - which we all would support," she said. "But to say that's the painless answer for more government spending betrays a lack of seriousness or perhaps even ignorance of the process."