Some of my fellow Republican candidates for the presidency have proposed plans to fix the tax system. These proposals are a step in the right direction, but the tax code has grown so corrupt, complicated, intrusive and antigrowth that I’ve concluded the system isn’t fixable.

So on Thursday I am announcing an over $2 trillion tax cut that would repeal the entire IRS tax code—more than 70,000 pages—and replace it with a low, broad-based tax of 14.5% on individuals and businesses. I would eliminate nearly every special-interest...