The Obama campaign released a new TV ad this morning, slamming Mitt Romney’s tax plan as a promise to eliminate popular deductions and raise taxes on the middle class.

“Mitt Romney: he won’t reveal what’s in his taxes and he won’t tell you what he’d do to yours,” the narrator darkly says. “To pay for huge new tax breaks for millionaires like him, Romney would have to raise taxes on the middle class. $2,000 for a family with children, says a non-partisan report. You could lose the deduction for your home mortgage, college tuition, health care.”

The ad concludes: “How much would you pay? Romney just won’t say.”

The Romney campaign has slammed the report referenced in the ad by attacking the source — the non-partisan Tax Policy Center — as flawed and biased.

The new ad reveals the weakness of that strategy, however; as long as Romney won’t explain which deductions he would eliminate to make his tax plan add up, he has no authoritative way to stop the Obama campaign from laying out their own plan in Romney’s name.