Around the world, governments in recent decades have sought to lighten the burden on capital by reducing taxes on dividends, capital gains, corporate profits and wealth. The motivation is straightforward: more capital means more investment, higher productivity and faster growing wages. Capital is also highly mobile: Tax it too much, and it will go elsewhere, undermining growth.

Massachusetts senator and Democratic presidential contender Elizabeth Warren has broken with that consensus by proposing a tax of 2% on net worth above...