CONCORD, N.H.—Sen. Bernie Sanders won the New Hampshire Democratic presidential primary Tuesday night in a narrow victory that ensures the race to challenge President Trump this November will remain heated.

With more than 85% of precincts reporting, Mr. Sanders, who is from neighboring Vermont, had 25.7% of the vote, followed by Pete Buttigieg with 24.4% and Sen. Amy Klobuchar of Minnesota with 19.7%.

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