Making the Tax Cuts and Jobs Act permanent would be a good start for Congress in 2025. But it’s time to take an even bigger step: creating a simplified flat tax for all. The TCJA got us close to this goal, and a few additional reforms would get it done. Collapsing the personal-income and corporate tax rates to 15% would have huge economic benefits. America would suddenly have one of the lowest tax rates in the world, resulting in trillions of dollars of new capital flow and a spike in take-home pay. The simplicity of a flat tax would reduce...