The top-earning 1% of U.S. households—and especially the top 0.1%—have a lot to lose from Hillary Clinton’s tax proposals.

The Democratic presidential nominee would concentrate her tax agenda on fewer than 1.5 million households by adding layers of taxation and sealing gaps that let some of their earnings get taxed relatively lightly. She would impose the highest marginal tax rates on top earners since the 1986 revamp of the tax code and boost taxes on business income, investments and estates. She would cap tax breaks while...