The TPC defends
The TPC has three lines of defense of its assumption on the growth effect of tax reform - firstly, they point to difficulties in estimating such effects; secondly they cite studies showing such effect will be small; thirdly they claim their analysis is robust even if some such growth is assumed.
We have drawn the Tax Policy Center's arguments on this score from their original study of the Romney proposal and from a later FAQ and Responses document. The complete section addressing this matter from the original TPC paper is cited below.