Members of Congress will always have an the incentive to raise the tax rate of one group to lower the rate of their supporters. I therefore find that option of intellectual interest only. The Flat Tax. But, unlike any other proposal, a flat tax introduces an element of sustainability into the equation. The key is the single rate.
All taxpayers can understand it, and all have a stake in it.
Second, the flat tax would end most of the existing differential treatment of capital and labor. That would likely include capital gains. With a flat rate in the low to mid twenties, it would mean a modest increase in the capital gains tax rate. Such an increase, however, would greatly assist in making the old-style distributional tables look more neutral. One could also imagine limiting the exemption to labor income.