Treasury Secretary Steven Mnuchincalls the Trump administration’s tax proposal “the largest tax reform in the history of our country.” The plan would slash corporate tax rates from 35% to 15% and roll back increases in individual rates that occurred under Presidents Clinton and Obama.

The announcement represents a first step toward a White House budget proposal that combines the president’s fiscal plans with reforms to defense spending and domestic policies including ObamaCare. If such a budget is passed, it would stimulate...