5 things to watch in the South Carolina debate: Will Biden's firewall hold?

Bernie Sanders braces for a pile-on, Elizabeth Warren and Pete Buttigieg fight to stay alive, and the former vice president faces a do-or-die moment at Tuesday night's faceoff.

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By Sahil Kapur

WASHINGTON — This time, Democrats are planning to focus their fire on front-runner Bernie Sanders, not Mike Bloomberg.

Six days after Bloomberg, the former mayor of New York, faced a less-than-warm welcome from rivals during his first time on the debate stage in Las Vegas, there's a new target on their minds: the man who crushed his nearest rival by more than 25 points in Nevada and demonstrated his broad appeal in the party.

Former Vice President Joe Biden, who came second in the Silver State, has to prove his strength in South Carolina or it could all be over for him. It's the first majority-black electorate on the calendar, and every Democratic presidential nominee since 1992 has won African American voters. So far, Biden has been their preferred candidate, but his support has shrunk after poor showings in early states.

And former South Bend, Indiana, Mayor Pete Buttigieg and Sen. Elizabeth Warren, D-Mass., two favorites of white college graduates, have yet to prove they can win black or Latino voters. They loom large in South Carolina on Saturday and the 14 states that will vote three days later on Super Tuesday, when about one-third of the delegates to the Democratic National Convention will be awarded.

Here are five things to watch in the last debate before South Carolina and Super Tuesday:

A Bernie Sanders pile-on?

Sanders' dominant victory in Nevada cemented his front-runner status in the field, and the Charleston debate is one of the few remaining opportunities for rivals to weaken him. In recent days, Democratic candidates have telegraphed their intentions to attack him from every direction. Biden and Bloomberg are faulting him for past comments praising Fidel Castro's literacy program, Buttigieg is calling him divisive and hammering his "Medicare for All" plan, Warren is criticizing him for wanting to preserve the Senate's 60-vote threshold and so on.

Sanders, the independent senator from Vermont, was somewhat spared being the focus of the Las Vegas debate because of Bloomberg's debut on the stage, but he probably won't be so lucky Tuesday in Charleston. The debate is a test of whether he can maintain his momentum after a popular vote win in Iowa and outright victories in New Hampshire and Nevada. A victory in South Carolina could make him virtually impossible to stop on Super Tuesday, when two of the most delegate-rich and Latino-heavy states, California and Texas, will vote.

That's due in part to one source of strength that's making rivals particularly nervous: Sanders thumped them by more than 30 points among Latinos in Nevada.

Will Biden's 'firewall' hold?

It's do-or-die time for Biden, the longtime front-runner who saw his support crater after he finished fourth in Iowa and fifth in New Hampshire. He finished second in Nevada, far behind Sanders, but with a silver lining: He won black voters by 9 points in the first primary contest with a diverse electorate, according to an NBC News entrance poll.

Two polls released Monday show that his firewall is still holding. Biden led Sanders by 4 points in an NBC News/Marist poll and by 15 points in a survey by the Democratic firm Public Policy Polling. He led by double digits among African American voters, who will make or break his prospects in the Palmetto State and the Super Tuesday contests three days later.

Biden's campaign downplayed Iowa and New Hampshire because they have paltry black populations. Biden needs to win South Carolina. If he does, he'll have a strong case for being the most viable moderate alternative to Sanders. If he falls short, there won't be any excuses left.

Will Mike Bloomberg pull it together?

The billionaire entrepreneur learned Wednesday that money can buy him limitless exposure in the form of slick ads but that it can't buy him a good debate performance in an uncontrolled environment. He was brusque at times and showed flashes of irritation as rivals pounded him on everything from attempting to buy the nomination to non-disclosure agreements signed by women who have worked for him. After rejecting Warren's demand that he release the women from their NDAs, Bloomberg reversed himself two days later, signaling that he was unprepared for the onslaught of criticism.

People close to Bloomberg say he was out of practice, participating in a debate for the first time since his last mayoral campaign in 2009. (He left public office in 2013.) Voters may forgive one poor debate performance as a fluke. A second one in Charleston could cause them to question whether the man they see blitzing the airwaves in TV ads reflects the candidate they'd be putting onstage with President Donald Trump.

Will the Warren bump materialize?

Warren delivered a fiery debate performance Wednesday in Las Vegas, telling Democrats why they should reject Bloomberg, Biden, Buttigieg and Sen. Amy Klobuchar of Minnesota and giving progressives a reason to choose her over Sanders. It came too late to save her in Nevada, where 71 percent of Democrats had locked in their decisions during the early-voting period that ended before the debate. One post-debate national poll by CBS News found her jumping to second place in the Democratic contest, with 19 percent, behind Sanders' 28 percent.

Warren continues to struggle with single-digit showings in two recent South Carolina surveys, and the onetime front-runner is running out of opportunities to prove to Democrats that she's their best option. A poor finish in South Carolina could take the wind out of her sails ahead of Super Tuesday. Charleston is her chance to turn her fortunes around.

Who's talking to Super Tuesday states?

Even before the Nevada caucuses, many of the candidates had begun campaigning in the cluster of states that will vote March 3 — Alabama, Arkansas, California, Colorado, Maine, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee, Texas, Utah, Vermont and Virginia. It's arguably the most important day on the calendar; watch for the candidates to use the Charleston debate to speak to this massive swath of voters.

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It's especially important for Bloomberg — who chose to skip the early states while his rivals were competing heavily there — instead using his personal fortune to run TV ads and his time campaigning in the delegate-rich March 3 states.

Then there are the two underdogs: Klobuchar and businessman Tom Steyer. Some Democratic elites argue that if they stay in the race through Super Tuesday and divide votes with other candidates, it could prevent consolidation around a non-Sanders alternative, allowing him to build an insurmountable lead. Keep an eye on how they perform in the debate — their performances may give a hint as to whether they intend to stick around.