Not only that, but a flat tax is a terrible idea! There are myriad ways to greatly simplify the tax code or to completely overhaul it. But there is no worse idea than a flat tax. There are good proposals out there. In the meantime we need to put progressiveness and more brackets back into the U.S. income tax structure. Sanders is correct about that.
This:
https://fairtax.org/about/how-fairtax-works is an interesting idea, but it incorporates no mechanism to prevent the distortion in wealth distribution from continuing to compound. It will result in worsening the distortion over time. That will lead to social unrest and a stagnating economy. Currently too little capital is remaining with the lower economic classes. Any tax proposal that does not address this problem, but instead makes it worse, will lead to trouble.