First, inequality undermines growth by over-concentrating spending power in too few hands — and slow growth is the single biggest threat to America’s fiscal future. Projections of future deficits vary substantially based on growth assumptions. Stronger growth equals more income to tax and lower deficits. Weaker growth means lower revenues and higher deficits. So if you buy the argument that siphoning so much national income into the nest eggs of rich people who don’t spend this money is bad for growth, you also have to buy the argument that inequality helps drive our fiscal problems.