Former Secretary of State Hillary Clinton, Vermont Sen. Bernie Sanders and former Maryland Gov. Martin O’Malley will face off in the NBC News-YouTube debate in Charleston, S.C., on Sunday, just 15 days before the first presidential contest in Iowa.

Polls show a tightening race between Mr. Sanders and Mrs. Clinton, while Mr. O’Malley remains an afterthought for most Democratic voters in state and national polls. This week’s debate remains among the last chances to shake up the dynamics of the Democratic primary.

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