Mitt Romney’s pledge to cut taxes for the middle class has been a cornerstone of his presidential campaign, even though he, notably, has never explained how he would make that happen.

But on the eve of the first presidential debate of the season, the Romney campaign apparently realized the Republican presidential candidate needs an actual plan. On Tuesday, Romney told a Denver, Colo., Fox affiliate that one option he was considering – he didn’t name any others – to pay for his tax plan would involve limiting tax deductions to $17,000, a move that would ostensibly rebuff arguments that his fiscal policies are specifically designed to benefit the wealthiest Americans.

“You could use your charitable deduction, your home mortgage deduction, or others – your health-care deduction,” Romney said.  “And you can fill that bucket, if you will that $17,000 bucket that way. And higher income people might have a lower number.”

But while including a cap on deductions would certainly raise more revenue under Romney’s proposed tax plan, his campaign has yet to specify just what deductions would be covered in that cap. For instance, will it include the state and local deduction? Is the cap in addition to, or instead of, the standard $5,700 deduction for single adults? Will individual taxpayers have a lower cap than families?

Even without knowing the details of Romney’s latest proposal, it is unlikely the GOP nominee’s tax plan would raise enough revenue to pay for its proposed cuts, while also managing to cut the federal deficit. William Gale, the co-director of the nonpartisan Tax Policy Center, told The Washington Post that the net revenue under Romney’s plan would reach a maximum of $2 trillion – far less than the approximately $5 trillion worth of tax cuts he is proposing.

When pressed for details, Romney campaign spokeswoman Andrea Saul was unable to say which deductions specifically would be included in that $17,000 figure.

“Governor Romney's tax reform plan will jump-start economic growth, cut the tax burden on the middle class, and lower tax rates across-the-board.  He will pursue revenue and distributional-neutrality in reforming the tax code.  There are a range of policy options, Gov. Romney referenced one illustrative example, to achieve these goals,” Saul said in an email.

But, so far, Romney’s tax plan is far from reaching that goal of “distributional neutrality.” As the Tax Policy Center has already reported in detail, the Romney tax plan -- which aims to reduce tax rates by 20 percent while also eliminating tax deductions that overwhelmingly benefit the middle class, such as the earned income tax credit -- would raise taxes by hundreds, if not thousands of dollars, on about 95 percent of Americans. Only those earning $200,000 or more would see a tax cut.

Because Romney has not specified how he would increase the tax base, the Tax Policy Center reports it is “impossible to determine how the plan would affect federal tax revenues.” If he does not lower his ideal 20 percent rate deduction, Romney’s plan must either add to the federal deficit or raise taxes on the middle class, according to the organization.

When confronted about the specifics of the tax plan, both Romney and his running mate, Rep. Paul Ryan, have sidestepped the question, telling interviewers they would negotiate those details with Congress after the election.

Romney’s tax plan will almost certainly be a topic of discussion during Wednesday’s presidential debate. In a memo and accompanying video released early Wednesday, Obama Deputy Campaign Manager Stephanie Cutter alleged Romney “doesn’t have a specific plan to move us forward” and challenged the GOP candidate to use the debate to “finally, for the first time, explain his proposals or readjust his positions.”