Democrats now have a big opportunity to design and advance a progressive economic agenda that could become a reality after 2020. Freshman Rep. Alexandria Ocasio-Cortez’s comments about a 70 percent tax rate on the wealthy set off a controversy over who does and doesn’t understand how America’s tax system works. What has been lost in the dust-up is that the wealthy pay relatively little in taxes compared with their peers in other developed countries, and the congresswoman is correct that they...