The Great Hippo wrote:Apparently, the issue was that they projected a 2 trillion dollar growth, which they then used to give the richest Americans a tax cut -- a tax cut that they claim is the source of that projected 2 trillion dollar growth. They're justifying their projections by using those projections to pay for the things that will cause those projections to be accurate. Kind of like saying "I'm going to make tons of money by investing in this company with all the money I'll make after I invest in this company". You can't write off the cost of an investment with the projected growth of that investment.
That's not precisely it. They argued the tax cut would be revenue neutral due to the growth. So the $2 trillion lost in taxes over the 10 years will be offset by the $2 trillion in growth. That's a fairly bold assumption but it's not the "mistake" they're talking about. You said "you can't write off the cost of an investment with the projected growth of that investment" but they do that all the time in budgets where growth is assumed.
The actual error is the budget also calls on the same $2 trillion in growth to pay for spending over the next 10 years. The way it was presented was the opposite, but the effect is the same. They said "our tax cuts are going to fuel economic growth which will let us have a balanced budget in 10 years". The problem was, the tax cuts needed that growth to pay for themselves, so basically you're double-counting the growth. Which means you'd need $4 trillion in growth to cover both the tax cuts and the spending.