Americans go to the polls next week facing what millions believe is the worst presidential choice of their lifetimes. As we wrote after Donald Trump won the Indiana primary in May, the New Yorker and Hillary Clinton are both deeply flawed. But one of them will be the next President, so in the next two days we’ll try to summarize the risks—and the fainter hopes—of each candidacy in turn.

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Start with Mrs. Clinton because the costs of her Presidency are easier to see in advance. To wit, she would continue President Obama’s...