In May 2014, a broad collection of thinkers and politicians gathered in Washington to celebrate a new conservative “manifesto.” The document called for replacing stodgy old Reaganite economics with warmer, fuzzier handouts to the middle class. Donald Trump must have missed the memo.

The president formally opened the tax wars on Wednesday with his speech in Missouri challenging Congress to meet his principles for reform. The media almost uniformly applied to the speech its favorite (though misused) descriptor: “populist.” But...