The concept of a flat tax is a often pushed by conservatives and libertarians. If you Google "flat tax" you'll find about 1.1 million hits, many from groups like the Heritage Foundation and Hoover Institution. A primer on the flat tax is on Wikipedia, but as on all online (and other) sources, read with a jaundiced eye. It is important to note that Rasoul's plan is actually a graduated flat tax which has some aspects of a flat tax while retaining several progressively higher tax rates as incomes increase. This compromise is probably more politically palatable than a pure flat tax.
While I'm not ready to endorse a flat tax, or even a graduated flat tax (simplification yes!), I applaud Rasoul for creative thinking and putting tough issues like this one on the table. That alone puts him head and shoulders above his opponent. This should be a topic in their upcoming debate and in future debates that we all hope the incumbent will agree to.