Washington ( CNN ) -- For months , voters have been in the dark about key details of Mitt Romney 's tax plans . He specified $ 5 trillion in tax cuts , a 20 % cut in income tax rates , a 40 % cut in the corporate tax rate , repeal of the estate tax and alternative minimum tax and elimination of taxes on interest , dividends and capital gains for households with incomes below $ 200,000 . He did not want his changes to raise the deficit , but he was utterly mum on how to raise $ 5 trillion to offset the tax cuts . During the summer , two colleagues and I showed that if Romney did not want to add new taxes on savings and investments -- and raising savings and investments is the second of four main planks in Romney 's overall economic package -- he could not finance his tax cuts without generating a net tax cut for households with income above $ 200,000 . Politics : 5 things we learned from the presidential debate . Even if all the available tax expenditures were closed in the most progressive manner possible , it would not raise enough revenue among high-income households to offset the tax cuts they would receive . This was true even when we adjusted the revenue estimates to allow for the impact of potential economic growth , and even when we gave the campaign a trillion-dollar mulligan by ignoring the cost of the corporate tax cuts . As a result , we concluded that if Romney did not impose new taxes on savings and investments , the only way to finance his tax cut proposals and reach revenue neutrality was to raise taxes on households with income below $ 200,000 . This was not a forecast of what Romney would actually do ; it was simply a matter of arithmetic . But it highlighted the need for specifics ; $ 5 trillion is not a trivial amount , even in Washington , and the prospect of middle-class tax increases sets off alarm bells . Earlier this week , Romney finally started the process of proposing ways to pay for his tax cut proposals . He broached the idea of putting a cap on each taxpayer 's total amount of itemized deductions -- including mortgage interest , state and local taxes , charitable contributions . Although critical design features remain foggy , Romney has said the cap could range from $ 17,000 to $ 50,000 , and it could vary with income . Several things are already clear . Opinion : Why you should vote for Romney . First , capping -- or even eliminating -- itemized deductions will not come close to paying for Romney 's tax cuts . It would be a step toward financing , but much more will be needed . Nevertheless , as a piece of the revenue puzzle , a cap is an interesting and important idea and a welcome step forward . Members of Congress are quick to see the political advantages of a cap . Relative to curtailing specific deductions , a cap allows them to leave existing deductions in place but restrict the overall use of such deductions . In that sense , the cap is like the alternative minimum tax was intended to be -- a limit on the overall use of tax shelters , even if political leaders could not shut down each one . A cap on itemized deductions goes after one of the three areas of the income tax where the money is . The other two are the exclusion of health insurance premiums from taxation and saving and investment incentives like 401 ( k ) plans , and the lower tax rates on capital gains and dividends and carried interest . A cap on a taxpayer 's use of all of these subsidies -- as opposed to just itemized deductions -- could get at all three areas . Martin Feldstein of Harvard University and the Romney campaign and Maya MacGuineas of the Center for a Responsible Federal Budget have proposed a different style of cap that applies to more than just itemized deductions . While Romney 's cap appears to apply to all itemized deductions , it may have a disproportionately negative effect on charitable contributions . After all , people have to pay their state and local taxes , and many people are already in the middle of a long-term commitment to pay down their mortgage . Opinion : Romney shakes up the race . For those households , there may be little room left under the cap to take deductions for charitable contributions . And , for all households , the cap would eliminate tax deductions for contributions larger than the cap , so large gifts to charities would automatically lose their tax-preferred status . So , a cap is not a panacea , but it could well be one part of a constructive solution . Likewise , his acknowledgment that his earlier , disparaging comments about the 47 % of households that do not pay federal income taxes were misguided suggests a reconsideration of the role taxes play in those households . If the journey of a thousand miles begins with a single step , Romney has finally taken the first step . But there is still much more work to be done . The opinions expressed in this commentary are solely those of William Gale .
William Gale : Mitt Romney 's $ 5 trillion tax cut proposal did n't add up for months . He says new idea of a cap on deductions is a first step toward a viable plan . Comment: He says that it could make taxpayers much less likely to give to charity .