Sympathetic economists such as Harvey S. Rosen of Princeton and Martin Feldstein of Harvard provided much more substantive responses to the TPC report, arguing that a tax reform plan similar to Romney's could indeed work. Their analyses differed from the TPC's in various ways, including the tax data they used (historical vs. projected), their assumptions about how tax reform would affect economic growth and the deductions they deemed to be "on the table."
But the most important factor in all these studies seems to be the definition of "high-income" vs. "middle-income" households. The TPC used a cutoff of $200,000, while Rosen and Feldstein preferred $100,000. (Rosen also ran the numbers based on the higher threshold, with results considerably less favorable to Romney.)
As The Washington Post's Dylan Matthews pointed out, the broader definition of middle-class households is counterintuitive, to say the least, since it applies to 96 percent of Americans. The narrower definition, by contrast, makes sense if the middle class corresponds to the middle fifth of household incomes. The problem for Romney is that he (like Obama) uses the broader definition, which is politically useful but fiscally inconvenient.
Similarly, Romney's lack of specificity avoids the risks of identifying which popular deductions he would target, but at the cost of letting his critics fill in the details. More important, his plan is backward as well as vague. Serious reform would start by eliminating the arbitrary, meddlesome and economically distorting complications that have made the tax code such a headache-inducing mess and only then ask how much rates should be lowered to keep revenue about the same.