Hillary Clinton is in a dead heat with Donald Trump in Florida and holds a six-point lead in North Carolina, according to new polls that give a sense of the cushion Clinton could use to absorb any fallout from a Federal Bureau of Investigation review of Clinton-related emails.

The Wall Street Journal/NBC News/Marist surveys, conducted Tuesday and Wednesday, found Clinton leading by a single percentage point in Florida, 45% to Mr. Trump’s 44%. That margin was narrower than the three-point lead she held a month ago.