The simplicity of a single rate might be the most appealing aspect of the flat tax to taxpayers, but the majority of its benefits would come from broadening the tax base, lowering rates and removing disincentives to save and invest. The biggest downside is that by sharply reducing the rates collected from the highest earners, it would force the lower and middle classes to cover more of the cost of government than under the current system of graduated rates.

Making the flat tax an option, as Gingrich and Perry favor, is a worst-of-both-worlds approach. It wouldn't reduce taxpayers' paperwork and expenses; they'd still need to calculate how much they'd owe under the current system to see whether they would pay less with the flat tax. Instead, it would let lower-income taxpayers and the elderly hold onto the valuable tax breaks that the flat tax would eliminate, while letting higher incomes take advantage of the flat tax's lower rates. In other words, it's status quo for those with lower incomes, but a boon to the wealthy.