Democratic presidential candidateHillary Clinton would levy a 65% tax on the largest estates and make it harder for wealthy people to pass appreciated assets to their heirs without paying taxes, expanding the list of tax increases she would impose on the top sliver of America’s affluent.

The estate-tax increase and other new proposals that Mrs. Clinton detailed on Thursday would generate $260 billion over the next decade, enough to pay for her plans to simplify small business taxes and expand the child tax credit, according...