5 Things to Watch for at Tonight's Democratic Presidential Debate

PHOTO: Pictured (L-R) are Democratic Presidential candidates Sen. Bernie Sanders in Washington, Nov. 19, 2015, Hillary Clinton in Salem, N.H., Dec. 8, 2015 and Martin OMalley in Manchester, N.H., Nov. 29, 2015.PlayGetty Images
WATCH Clinton Accuses Sanders of Flip-Flopping on Guns

Get ready for a showdown.

For the first time in 2016, all three Democratic candidates -- Hillary Clinton, Bernie Sanders and Martin O'Malley -- will take the same stage at the same time, for a debate tonight in Charleston, South Carolina.

With just 15 days until the Iowa Caucus, and the polls tighter than ever, here are five things to watch out for as the three face off in the fourth Democratic presidential debate, hosted by NBC News:

1) FIREWORKS OVER HEALTHCARE: Despite serious backlash from progressives, Clinton has doubled down on her critique of Sanders' Medicare-for-all, single-payer health care proposal. The Vermont senator has fired back, however, with his campaign calling the former secretary of state a flip-flopper on the issue and releasing old photos and videos of her talking about universal health care. We expect the two to go toe-to-toe on this one on the main stage.

2) A TANGLE ABOUT WALL STREET: The Sanders campaign released a new ad this week in which the senator says: "There are two Democratic visions for regulating Wall Street. One says it's OK to take millions from big banks and then tell them what to do. My plan: break up the big banks." The Clinton campaign immediately accused Sanders of reneging on a promise not to run negative ads. The senator has stood by his ad and continues to argue that his decision not to take money from Wall Street, as well as his plan for breaking up the country's largest banks makes him tougher on this issue than her. Will she concede on that? Not likely.

3) TRUMP AS THE 'PUNCHING BAG': Although Clinton made a New Year's resolution to stop responding to him, the businessman has been a mainstay of Democrats' previous debates. The candidates have used his name as a rallying cry aimed against Republicans and often go so far as to call him a racist and a demagogue. Considering he's still the leading Republican, his name is likely to be bandied around tonight.

4) O’MALLEY'S VALIANT, LAST DITCH EFFORT: There were serious questions as to whether the former Maryland governor would even make this debate after NBC announced polling benchmarks for entry, but O'Malley inched in, just barely. Now tonight, he's likely to give it all he's got in an effort to finally break through in what has become a two-person race.

5) IF ANYONE EVEN WATCHES: Not able to watch? You're probably not alone. The Democratic National Committee has come under fire for the debate schedule this season. The last two Democratic debates were held on Saturday nights, and tonight's debate is on a Sunday smack in the middle of a holiday weekend. Viewers may be few and far between because of it.