The brawl in President Obama’s second term over raising the top income-tax rate to 39.6% from 35% was centuries ago in political time. One way to tell is that even moderate Democratic 2020 presidential candidates have quietly proposed to raise the tax rate on labor by double digits and it’s received almost no attention. Unlike single-payer health care and wealth taxes, this tax increase could command majority support in a Democratic Congress on day one.

The idea is to increase the Social Security payroll tax, the 12.4% levy...