And whenever flat-tax theory has been translated into real-world proposals for the U.S., the results have been problematic. Flat-tax proposals by Newt Gingrich and Rick Perry during the 2012 Republican presidential campaign slashed federal tax revenues by nearly $500 billion to $1 trillion a year, which should give pause to even the most fervent supporters of dynamic scoring. Making such plans revenue-neutral, when top rates would fall on wealthy Americans, would mean raising taxes on someone else — such as millions of middle-to-low income Americans. The Heritage Foundation would replace income, payroll, and excise taxes with a 28 percent flat tax. It claims the plan would leave the distribution of the tax burden unchanged, but the proposal would also raise revenue of just 18.5 percent of GDP.

Then there’s the uncomfortable political reality that the flat-tax concept has never been popular with voters. A 2011 Wall Street Journal survey found that not even a majority of GOP voters favored the idea. A poll by the Hill the same year found 58 percent of Americans favored a graduated income-tax system, about the same as the Journal poll.