Romney says his tax plan adds up, assuming that some revenue to pay for the proposed tax cuts comes from growth. Obama, in contending the plan doesn't add up, cites math used by Congress, which doesn't allow higher growth to cover the cost of tax cuts.
The nonpartisan Tax Policy Center said that, even with growth effects, Romney's plan would be $33 billion short of paying for itself in 2015. Romney allies objected and said other changes, such as ending the tax break for municipal bond interest, would fill the gap.
Under the Tax Policy Center's standard, erasing that gap would require Romney to support proposals he hasn't offered or has suggested he wouldn't accept, such as eliminating the charitable deduction once a household's annual income reaches $200,000.
Another variable in any tax overhaul is how to classify carried interest, or the share of profits that private-equity managers receive.
Carried interest is now taxed as capital gains -- at 15 percent -- rather than at higher rates for ordinary income. Obama wants to tax it as ordinary income, while the Romney campaign hasn't said whether he wants to keep the preferential tax treatment.
"It's income, without a doubt," Hufbauer said. Hodge said that while it "is probably income, if you eliminated it, people would find other ways to accomplish the same thing."