Nineteen uberwealthy Americans posted an open letter Monday calling on “all candidates for President” to support a “moderate” wealth tax. Signatories include the investor George Soros, Berkshire Hathaway scion Molly Munger, Mickey Mouse heiress Abigail Disney, Facebook co-founder Chris Hughes, and a couple of Hyatt Hotel progeny from the Pritzker family.

“America has a moral, ethical and economic responsibility to tax our wealth more,” they say. Revenue squeezed from the top 0.1% could fund “smart investments,” such as “clean...