Hillary Clinton got out of Iowa with a narrow win. She explained away her crushing loss in New Hampshire. But what happens if she can’t prevail in Nevada?

The Silver State was once considered part of Clinton’s firewall against the encroaching Bern. But it appears—that’s the key word here—that Senator Sanders has closed that gap, making the state a toss-up. Clinton backers are now just including Southern states with large African American populations in the firewall, and insisting they always expected Nevada to be close. On the eve of the caucuses, Nevada looks like all upside for Sanders. If he wins, it will be the latest blow to the argument that Clinton will win the Democratic nomination because Sanders can’t win minority voters.

If he loses in a close race, it’s still a major victory for Sanders. Clinton started staffing up there in August 2015; Sanders was two months behind. The Clinton family has deep roots in the state. She won the race in 2008—although, thanks to quirks in delegate-allocation rules, Barack Obama still won more delegates. But Clinton’s edge has been battered by forces in and outside the state. Outside, Sanders has all the momentum, and he’s been working hard to catch up with among minority voters. He has spent much of the last week campaigning across the state. Late Thursday, he was endorsed by the black caucus in Clark County, home to Las Vegas. Meanwhile, Senate Minority Leader Harry Reid, a towering figure in the state, has tacitly backed Clinton but hasn’t publicly endorsed her, part of an elaborate and classically Reidian ploy to increase turnout. In short, Nevada wasn’t supposed to be close.

Assuming, of course, that it is close. No one is totally sure. The polling in Nevada is sparse and thought to be unreliable, making it tough to know exactly what the situation is. RealClearPolitics includes just three polls so far this year in its average, which show the two candidates either even or Clinton a little bit ahead. Part of the difficulty springs from the process—the fact that Nevada uses a caucus rather than a primary—and from the state’s geography, which is more spread out and wide open than the other early states, as Nora Kelly reports. Plus there's same-day registration for caucuses, meaning that Republicans and independents could show up Saturday, register as Democrats, and participate.

One reason that Nevada has the nation’s fourth Democratic contest, and one reason it’s expected to be such an important one, is that the state has a far less white Democratic electorate than Iowa and New Hampshire. In 2008, 15 percent of caucusgoers were black and another 15 percent were Hispanic, with whites accounting for 65 percent. The premise of the Clinton firewall argument is that Sanders would run into trouble once he left rural, white states like Iowa and New Hampshire behind and had to content with voters of color. But lately, Clinton’s backers are talking more and more about how that will play in the south, and in particular in South Carolina, than they are in Nevada. Both campaigns have been making their play for Hispanic voters.

On Thursday, Clinton released what’s probably her strongest spot of the cycle, which is just raw footage of a girl whose parents could be deported speaking to her at a rally:

The same day, Sanders released his own strong video, a bilingual ad focusing on the housing crisis, which hit Nevada hard. (Don’t miss Sanders’s Spanish-language disclaimer at the end.)

Both campaigns have also courted black voters. Clinton has tied herself closely to President Obama and accused Sanders of betraying him. In an interview with BET, Sanders accused her of pandering: “We know what that's about. That's trying to win support from the African American community where the president is enormously popular.”

Besides the minority vote, the two campaigns have also battled for the backing of the Culinary Workers Union. The Sanders campaign seemed to have committed a major gaffe when the union accused his staffers of impersonating union workers, but they somehow resolved the dispute, and the union has stayed out of the race.

Assuming the race is as close as the polls suggest, Sanders and Clinton will split a roughly equal number of the Nevada’s 43 delegates. That would be Clinton still way ahead of Sanders in the delegate count—though it remains hard to believe that superdelegates would deliver the nomination to Clinton if primary voters clearly favor Sanders.

Whatever the results in Nevada, they’re almost inseparable from what happens in South Carolina on February 27. If Clinton wins the Nevada caucus, and particularly if she wins with some room, she’ll go to South Carolina breathing a little bit easier, and she’ll arrive with a polling lead and the recent endorsement of Representative Jim Clyburn. But what if Sanders wins Nevada, and particularly if exit polls show him doing well with black and Hispanic voters? Clinton has a big lead in South Carolina, but one lesson of the last few months is that Clinton’s big leads are fluid and fleeting. There’s already been a shift of many black leaders and groups to Sanders, and if can prove that he can win in Nevada, why should anyone be sure the rest of the firewall will hold?