As far as I can tell, not much has been said about the possible effects of the Romney proposal on economic growth. This is curious because increasing growth is the motivation for the proposal in the first place.
....As I understand it, the TPC model is based on a large set of publicly available tax returns (in electronic form and anonymized) provided by the Internal Revenue Service. When analyzing any change in the tax code, the TPC in effect plays H&R Block for every return....A not too veiled reference to the insight that made Mr. Block's success possible; most federal tax return problems are due to simple errors in arithmetic and many people are so limited intellectually that they're willing to pay someone to check for those.
But professional economists should be expected to be made of sterner stuff, they should be analysts;
An important complication arises if taxes affect people’s behavior.
[big snip of calculations]
For the over $100,000 group, the reduction in revenue because of rate cuts is about $144 billion; the increase in revenue due to base broadening is $200 billion; and with a 3 percentage point growth assumption, the additional revenue from a rise in incomes is $25 billion.
The net impact is a positive $81 billion. That is, under these assumptions, taxpayers with incomes of $100,000 or more would pay $81 billion more in taxes.So much for Prof. DeLong's 'mathematically impossible' claim.