CHARLESTON, South Carolina — Things could get ugly here Sunday night.

The fourth Democratic debate, which is also the first one of 2016 and the last one before voters finally weigh in, is shaping up to be the most heated and consequential face off yet.

With just 14 days to go before the Feb. 1 Iowa caucuses, polls have unexpectedly tightened between Hillary Clinton and Bernie Sanders, making the caucuses and New Hampshire’s Feb. 8 primary — and thus the entire nominating contest — look for the moment like a virtual dead heat. And as the gap between the candidates has closed, the fighting between them has intensified.

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Even though the candidates have faced off three times before, Sunday night’s debate, hosted by NBC News and the Congressional Black Caucus, may give voters the purest look at the differences between the candidates thus far.

The two previous debates were overshadowed by late-breaking news that came in the day before candidates met onstage (the Paris terror attack in November and the Democratic data breach in December), while the first debate in October came before the candidates had begun to draw sharp contrasts with each other.

An apology from either candidate seems less likely this time. In the previous two weeks, the Clinton and Sanders campaigns have exchanged fire on Wall Street regulations, electability, guns, health care, and whether a new Sanders TV ad qualifies as negative.

Here are four things to watch:

Sanders on defense

Sanders has yet to release the details of a tax plan he needs to fund his proposed single-payer health care plan, and his campaign has given conflicting answers about when it would come. The Vermont senator has been saying since at least July that it would come “soon,” and in recent interviews said he would present it before the Iowa caucuses, but the moderators and Clinton are likely to press him for details.

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Sanders worked to remove another piece of baggage the night before the debate, when his campaign announced he would support a bill to revoke the legal immunity granted to gun makers by a 2005 bill for which Sanders voted. Clinton has been hammering her rival for that vote, accusing him of doing the bidding of the National Rifle Association.

Clinton campaign chairman John Podesta suggested in a tweet that Clinton would not give Sanders a pass on what he called a “debate-eve conversion.” Expect the words “flip flop” to come up a lot as Clinton tries to paint Sanders as typical politician.

view photo essay Democrats face off in NBC News-YouTube debate The 2016 Democratic presidential candidates gathered for their first debate of 2016 Sunday night – and there were fireworks.

Gun fight

The debate venue, the Gaillard Center for the Performing Arts, is almost directly across the street from the Emanuel AME church where a shooter with neo-Confederate sympathies killed nine black worshipers last June.

Beyond the immunity law, Clinton has plenty of other fodder with which to attack Sanders on guns. That includes the so-called Charleston Loophole, which allowed shooter Dylan Roof to buy a gun, even though a previous legal issue should have prevented him. The Washington Post fact checker confirmed that Sanders supported that loophole.

Clinton overarch?

On health care, Clinton and Sanders have a deep policy difference that reflect a philosophical as well as strategic divide. Sanders supports a single-payer system, while Clinton has defended Obamacare, saying she wants to improve the president’s sweeping healthcare law.

RELATED: Sanders: Clinton ‘getting defensive’ as polls tighten

But in her campaign’s zealousness to attack Sanders this week, Clinton and her staff made a series of claims about Sanders’ proposal that many found tendentious. Vox’s Ezra Klein, who has earned respect as a progressive policy wonk, wrote that their argument had moved from “disingenuousness” to “farce.”

And even many Clinton allies have privately expressed doubts about the wisdom of campaign’s strategy to accuse Sanders of violating his own pledge not to run negative TV ads. Sanders’ campaign has touted that they’ve already raised more than $3.3 million from the attacks, according to aides, but whether he can score points against her on stage remains to be seen.

The O’Malley wildcard

It’s easy to forget there’s a third candidate in this race (Joe Biden and Jimmy Fallon recently did), but O’Malley will be on stage Sunday night and has proved a willingness to aggressively engage with the two frontrunners. In last month’s ABC News debate in New Hampshire, O’Malley interrupted every chance he got, forcing moderators to shut him down repeatedly. He could decide, however, to give a voters a warmer final impression of him in case he drops out after the Iowa Caucuses.