The next Democratic debate is on Sunday, January 17, at 9 pm Eastern. The debate will take place in Charleston, South Carolina, and air on NBC. A free online live stream will be available to all on NBC News's YouTube channel.

Like the last debate, this one will feature all three of the remaining Democratic candidates: Hillary Clinton, Bernie Sanders, and Martin O'Malley. It will be the fourth of just six debates that Democrats are planning. And, like the last two debates, it will take place on the weekend (a three-day weekend, in this case) — when fewer people are expected to watch.

Tempted as you might be to skip it, though, this will be the final Democratic debate before the Iowa caucuses and New Hampshire primaries kick off the actual voting in early February. So tune in now, before it's too late — especially because the Democratic race has just started to get interesting.

What to expect at the fourth Democratic debate

For much of 2015, it appeared Hillary Clinton would win the Democratic nomination without too much trouble.

Not anymore: Bernie Sanders has been surging in both Iowa and New Hampshire, the first two states to vote. He's been leading in the Granite State for most of the past few months, but his Iowa momentum is more recent — and the unique dynamics of the caucuses could give him an advantage over Clinton. Read my report from Sanders events in Iowa to learn more about what's happening there.

So Clinton is now faced with the possibility that she could lose both of the first two states to vote — something that would be hugely embarrassing for the supposedly inevitable Democratic nominee. It's unclear whether Sanders's success in Iowa and New Hampshire will translate to other states, but it's obvious Clinton has gotten nervous: This week, her campaign started lashing out at Sanders's support for a single-payer health care system — using attacks that many commentators dubbed misleading. Expect a great deal of discussion on the candidates' health care positions at the debate.

Another topic that will surely be discussed is electability. With the Republican nomination contest lurching so far to the right, many Democrats are anxious to nominate a candidate that will ensure their party keeps the White House this November. And some believe that Sanders — a "democratic socialist" who holds far-left views on several issues — wouldn't be able to win.

Clinton's campaign has been running ads suggesting she's the only candidate who can stop the GOP, while Sanders has responded by saying early polls show him doing better than she would. So it's likely that electability will be hotly debated on Sunday.

Finally, there's the core issue motivating Sanders's campaign and indeed his entire political career — his desire to check the power of the superwealthy and corporations. Sanders has been laser-focused on this for decades, while Clinton has been much more of an ordinary mainstream Democrat — willing to push for policies that improve people's lives, but also eager to win the business community to her side.

In a recent ad, Sanders said there are "two Democratic visions for regulating Wall Street," and that "one says it's okay to take millions from big banks and then tell him what to do." So expect a serious debate around whether Clinton is too close to the wealthy.

As for the other candidate in the race — Martin O'Malley, who's way far back in last place — this may be the last time we see him in a debate. If polls of Iowa are anywhere close to accurate, he'll perform so poorly there that it's difficult to imagine him continuing his campaign. Vox's Matthew Yglesias has argued that O'Malley should be taken more seriously — so this may be the last chance for that to happen.

How to watch

When: 9 pm Eastern

Where: Gaillard Center, Charleston, South Carolina

TV: NBC

Online: A free live stream on NBC News's YouTube channel