It has been nearly a year since President Trump signed sweeping tax changes into law. The macroeconomic data already rule out some of the more extravagant claims about immediate jumps in wages and capital. But the more serious debate over the tax cuts’ long-run impact is still far from decided. Here’s what we do know:

First, fiscal policy—including the tax cuts, increased federal spending, and increased state and local government spending—has clearly added to growth and jobs in 2018. Based on the latest tracking estimates,...