I do. For corporations. So no corporation can get out of paying taxes. You pay 20%, no loopholes.
And, to foster employment in the United States, there should be a penalty v bonus scale system, to reward companies that keep employment in the United States, and to punish companies that outsource plants and work.
Realize that even though corporate taxes in the United States are among the highest on the planet, as Republicans like to say on Fox News and to any organization that offers them a mic to spew spin at, that is true just in terms of statutory rates, not for effective rates. Many companies pay no tax. So you support a flat tax? Start by supporting a flat tax system for those who systematically use the law to pay no tax at all, and who use profits to destroy small businesses and export jobs abroad: large American corporations.