The left was pretty excited, and some tepid Republicans were pretty nervous, when the Joint Committee on Taxation came out last week with a supposedly "dynamic" forecast that showed even with likely economic growth factored in, the tax cut proposal would still add more than $1 trillion to the deficit over the course of a decade.
The reason? Joint Tax estimated the likely economic growth effect of the tax cut at only about $400 billion in new revenues to the Treasury.
Now: How would they know to that level of precision? They don't. But they base their expectation of growth on some very limited models, which completely fail to account for the impact of tax policy on the movement of capital in international markets.