Perhaps you’ve read that the federal budget deficit is rising again, and that’s true. But what you probably haven’t heard is that the main reason is spending, not falling revenue from tax cuts.

The Congressional Budget Office released its budget summary for July this week, and the deficit for the first 10 months of fiscal 2018 reached $682 billion, up $116 billion from a year earlier. Federal spending increased by $143 billion for all the usual reasons—especially Medicare, Medicaid and Social Security.

But...