The next Democratic debate is tonight at 9 pm Eastern time and will air on NBC. However, if you don't have a TV, don't despair — a free online live stream is available to all on NBC News's YouTube channel.

This debate — the fourth of just six overall that Democrats are planning — will once again feature all three of the remaining Democratic candidates: Hillary Clinton, Bernie Sanders, and Martin O'Malley.

If you think it's odd for this debate to be scheduled on the Sunday night of a three-day weekend, you're not alone. Amazingly enough, half of the Democratic debates were scheduled to take place on the weekend — when many fewer people are expected to watch.

Regardless, though, millions are likely to tune in for this one, because the Democratic contest has been heating up of late. As Sanders surges in the polls in both Iowa and New Hampshire, a nervous Clinton has gone on the attack over his support for single-payer health care.

Some of the arguments Clinton's used have been criticized as misleading by commentators, but the health reform issues gets to the heart of the differences between those two leading candidates: one is a bone-deep pragmatist obsessed with what can actually get done, and the other hopes to mobilize a political revolution to move US politics far to the left.

In addition to Clinton and Sanders, there's Martin O'Malley, who's been mostly overlooked so far, with no signs of that changing anytime soon. This may be his last chance to make a splash, as it's difficult to imagine his campaign surviving past the Iowa caucuses unless he does far, far better than expected there. (The slim bit of good news for him is that expectations for him are quite low.)

How to watch

When: 9 pm Eastern time

Where: Gaillard Center, Charleston, South Carolina

TV: NBC

Online: At NBC News's YouTube channel, a free live stream will be available