The House GOP’s reform proposal for individual taxes is a mess, but now we learn it also includes a stealthy 45.6% marginal tax rate on some high earners. This dishonest surcharge betrays the GOP’s purposes of growing the economy and simplifying the code, and Republicans ought to kill this gift to the left that will be slapped on more Americans when Democrats return to power.

The GOP’s tax bill released Thursday collapses seven brackets into four—12%, 25%, 35% and 39.6%. But the 12% rate applies to a large range of income,...