Hillary Clinton’s 2015 tax return showed a sharp drop in income as she launched her presidential campaign, along with a higher federal effective income-tax rate than any president since Gerald Ford, a sign of caution for a candidate facing intense scrutiny over her family’s finances.

The return showed Mrs. Clinton and her husband, former President Bill Clinton, made $10.6 million in adjusted gross income last year, largely from speaking fees, books, and Mr. Clinton’s consulting activities.

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