Here's the tax policy center's statement:
At issue is whether they assumed the right tax breaks would be eliminated. For example, there is another tax break that they could have chosen which is the non-inclusion of interest income from state and municipal bonds. As I understand it, this results in a large savings for upper income individuals. (Obama had proposed eliminating this for those earning over $250,000 but it didn't go anywhere.) There are $3 trillion of muni bonds outstanding, so the potential tax on the annual interest is a reasonably large number. I agree that it is now incumbent on Romney to fill in the details.
Our major conclusion is that a revenue-neutral individual income tax change that incorporates the features Governor Romney has proposed - including reducing marginal tax rates substantially, eliminating the individual alternative minimum tax (AMT) and maintaining all tax breaks for saving and investment - would provide large tax cuts to high-income households, and increase the tax burdens on middle- and/or lower-income taxpayers. This is true even when we bias our assumptions about which and whose tax expenditures are reduced to make the resulting tax system as progressive as possible. For instance, even when we assume that tax breaks - like the charitable deduction, mortgage interest deduction, and the exclusion for health insurance - are completely eliminated for higher-income households first, and only then reduced as necessary for other households to achieve overall revenue-neutrality- the net effect of the plan would be a tax cut for high-income households coupled with a tax increase for middle-income households. [Tax Policy Center, 8/1/12]
At issue is whether they assumed the right tax breaks would be eliminated. For example, there is another tax break that they could have chosen which is the non-inclusion of interest income from state and municipal bonds. As I understand it, this results in a large savings for upper income individuals. (Obama had proposed eliminating this for those earning over $250,000 but it didn't go anywhere.) There are $3 trillion of muni bonds outstanding, so the potential tax on the annual interest is a reasonably large number. I agree that it is now incumbent on Romney to fill in the details.