03/07/2012 12:43 pm ET

Mitt Romney: Tax Plan 'Can't Be Scored'

BOSTON -- Mitt Romney is acknowledging that it's impossible to know how his tax plan will affect the federal budget deficit.

In an interview Wednesday on CNBC, the former Massachusetts governor scoffed at outside groups who have said his plan to lower marginal tax rates would increase the deficit.

Romney said he was surprised that such assertions were being made because his plan "can't be scored." He says he'll have to work with Congress on the details before he can estimate how much the plan will cost.

“It is essential to me that we not place a larger share of the tax burden on middle-income taxpayers, so that means that we’re not going to end up with very high-income taxpayers taking a smaller share,” said Romney. “We’re going to have to limit the deductions and exemptions in such a way -- again, limiting them more toward high-income folks, so that high-income folks don’t pay a smaller share of the total, they continue to pay the same share they pay now, same thing with the middle-income folks.”

Romney previously has insisted that his plan to cut marginal individual tax rates by 20 percent won't increase the deficit because he would limit deductions and exemptions for the wealthy. He hasn't outlined what those changes would be, and says he'll work with Congress.

A study by the nonpartisan Committee for a Responsible Federal Budget said that Romney's plan would increase the national debt by about $2.6 trillion. A Tax Policy Center analysis said that the Romney plan would add $480 billion to the deficit in 2015 alone, and wealthier Americans would get more of a tax cut than middle-income earners.