For many Republican politicians and conservative wonks, “tax reform” has been something of a Holy Grail. “Cut the rates and simplify the code” has long been the mantra of folks like Paul Ryan, presidential hopefuls, and think-tank scholars.

There were dozens of models for reform: a flat tax, a national sales tax, nine-nine-nine, a consumption tax, and many others. They all aimed to stimulate growth by eliminating the distortions created by arbitrary, politically favored deductions, and to reduce the disincentive to work and risk created by high top rates.