By now readers have heard that progressive luminary Alexandria Ocasio-Cortez (AOC for groupies) supports a 70% top marginal tax rate, which she says will help finance a “green new deal.” Higher taxes on the rich is the stock socialist answer on how to pay for any project, though a reminder arrived this week that soaking the wealthy will barely register as a down payment.

The Tax Foundation on Monday did Ms. Ocasio-Cortez the favor of taking her proposal seriously and asked: How much money would the government reap from a 70%...