Sen. Bernie Sanders, former Secretary of State Hillary Clinton and former Maryland Gov. Martin O'Malley will face off Sunday in the fourth Democratic debate at the Gaillard Center in Charleston, South Carolina. NBC News, the Congressional Black Caucus Institute and the South Carolina Democratic Party are co-hosting the debate.

Viewers can watch the debate online on NBC's website and YouTube, with coverage beginning at 8 p.m.; questions for the candidates can also be submitted via YouTube, CBS News reported. Television networks will air the debate at 9 p.m., which is when the debate actually begins, according to NBC News.

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Clinton NBC/Getty Images

Sanders Andrew Harnik/AP

O'Malley Patrick Semansky/AP

NBC Nightly News anchor Lester Holt will be the principal moderator, and Andrea Mitchell, NBC News' chief foreign affairs correspondent, will be the co-moderator, NBC News reported. Meet the Press' Chuck Todd will be anchoring before and after the debate.

O'Malley just made the upcoming debate's cut, thanks to a roundup of NBC-recognized poll percentages, the New York Times reported. The minimum percentage for a candidate to participate in the Democratic debate is 5%, and O'Malley, who trails far behind Clinton and Sanders, roughly lingered at that rate in Iowa. On Thursday, NBC executives announced the good news for O'Malley. Even Sanders had logged onto Twitter to voice his support for O'Malley's participation.

In the last debate, the candidates discussed gun control, terrorism and Donald Trump's controversial Muslim ban.

The latest Bloomberg Politics/Des Moines Register poll, which came out Thursday, showed Clinton leading Sanders by just 2%. The Iowa caucuses are approaching Feb. 1. But first, the candidates must tackle Sunday night's debate.