While Republicans in South Carolina will spend today voting, Democrats in Nevada have a caucus that appears to be going down to the wire:

LAS VEGAS — In a storefront on this city’s heavily Latino east side, the civil rights leader Dolores Huerta rallied a dozen volunteers for Hillary Clinton on Wednesday night, relating in Spanish a Mexican saying about people who go near a cactus only when it is bearing fruit.

“Bernie hasn’t been around,” Ms. Huerta said, referring to Senator Bernie Sanders of Vermont, Mrs. Clinton’s opponent in Nevada’s Democratic caucuses on Saturday. “He came to see the Latinos when they had the fruit.”

Fifteen minutes away, the Mexican actor and game-show host Marco Antonio Regil stood in the backyard of a suburban home, urging on volunteers for Mr. Sanders and his vow to fight income inequality.

He recalled growing up in Tijuana, with its wide gap between the rich and the poor. “We cannot allow that extreme social disparity to happen in America,” Mr. Regil said.

The dueling pep rallies pointed to how hotly contested the race here has become — particularly among Latino voters. Nevada was once supposed to be a firewall for Mrs. Clinton, its large minority population primed to accelerate her drive to the Democratic nomination. But since her narrow victory in Iowa and crushing defeat in New Hampshire, it has turned into yet another tight and unpredictable contest, in which Mr. Sanders stands to gain more from a victory and Mrs. Clinton stands to lose more from a defeat.

A victory for Mrs. Clinton would not be as remarkable as a win would be for Mr. Sanders, given her history in Nevada and months of hard preparatory work. But a loss could raise concerns among Democrats about her viability. Mrs. Clinton’s aides have appeared to brace for the worst here, playing down expectations and shifting their attention to the South Carolina primary next weekend and on the 11 states that hold contests on Super Tuesday, March 1.

But after Mrs. Clinton led in polls here by more than 20 points a few months ago, the sharply altered outlook shows how her organizational muscle has encountered unexpected resistance from a well-funded and increasingly popular opponent.

And the results on Saturday, particularly among Nevada’s Latino population, could be a harbinger of outcomes in states like Texas and Colorado, which both have contests on March 1.

On Thursday night, Mr. Sanders and Mrs. Clinton both tried to reaffirm their support for immigration reform at a bilingual town hall-style forum held by MSNBC and Telemundo.

In the coming days, Mrs. Clinton will hold just one public rally per day, and will drop in on casinos to chat with employees. She plans to attend a rally in Houston on Saturday evening after the Nevada results come in. Few of her senior aides have entrenched themselves here as they did in Iowa and New Hampshire.

And Mrs. Clinton’s spokesman, Brian Fallon, even sought last week to portray Nevada’s caucusgoers as predominantly white and liberal, much like voters in Iowa and New Hampshire — suggesting this plays to Mr. Sanders’s advantage.

But focusing on turnout ignores demographic facts: Nevada’s population is nearly half minority, and Democratic caucusgoers in 2008 were 35 percent minority, exit polls showed.

“He must have been looking at one of my high school yearbooks, because things have changed dramatically,” said Senator Harry Reid, the Nevada Democrat who engineered the caucus system in 2008.

Indeed, Nevada’s diversity — its population is nearly 28 percent Hispanic, 9 percent African-American and 8 percent Asian — was a major part of Mr. Reid’s argument for making the state an early contest for the Democratic nomination.

Mrs. Clinton showed force early on here. Robby Mook, her campaign manager, had served as state director in 2008 when she narrowly defeated Barack Obama in the popular vote in Nevada. Mr. Mook in turn brought on Emmy Ruiz, who led Mr. Obama’s re-election efforts in Nevada in 2012, when he defeated Mitt Romney by nearly seven percentage points.

And Mrs. Clinton unveiled her immigration plan, which she vowed would go further than Mr. Obama’s had, at a high school in Las Vegas, with the children of undocumented immigrants sitting beside her.

By August, her campaign had 22 full-time staff members and had held more than 1,100 one-on-one meetings with Latinos, Native Americans, Asian-Americans and African-Americans.

Mr. Sanders did not have a single staff member in the state until October, and his state director unexpectedly quit a month later.

Joan Kato, who worked on Latino outreach for Mr. Obama in Nevada and other states in 2008, took over the Sanders operation in the state. But it was already looking like a lost cause.

That has not turned out to be the case.

Today, Mr. Sanders has 12 offices across the state, more than any other political candidate, with outposts in remote areas like Winnemucca, home to the Northern Paiute tribe, and Fernley, outside Reno. He has outspent Mrs. Clinton on advertising in the state by nearly two to one, and his campaign has sent out more multilingual door hangers and fliers than the Clinton campaign, according to Ralston Reports, a local website that tracks political spending.

“Did they build a firewall and leave it alone? Were they arrogant?” Jon Ralston, the site’s owner and a veteran Nevada political reporter, said of the Clinton campaign. “Call it whatever you want, but it’s clear that a state that they never thought would be in play is now in play.”

The Clinton campaign has been effective in Nevada nonetheless, local officials say, securing high-level endorsements and the support of labor unions and reaching out to diverse constituencies.

“You can’t just parachute in a month before the caucuses and have the kind of field organization she’s got,” said Representative Dina Titus, a Nevada Democrat who has endorsed Mrs. Clinton.

But Mr. Sanders’s small-dollar donor base has allowed him to outspend Mrs. Clinton here as her campaign poured resources into Iowa and, sensing a longer-than-expected nomination fight, began to ramp up in the states that vote in March.

Mr. Sanders’s win in New Hampshire gave his populist economic message momentum in Nevada, where unemployment is at 7.1 percent, compared with 4.9 percent nationally.

“All the organization is good for is one or five points, but beyond that it is really about the public mood,” said David F. Damore, associate professor of political science at the University of Nevada, Las Vegas.

Determining that public in Nevada, though, isn’t exactly easy. Because of the odd format in which the caucuses are conducted, including the fact that many caucus sites are being held on work sites such as in the Las Vegas casinos so that casino and hotel workers can participate without losing time from work, and the fact that one does not need to be registered with the party beforehand to participate, it’s been historically difficult for pollsters to predict exactly who a likely caucus goer is with sufficient accuracy. Because of this, the polls have often been off the mark in predicting the final result, such as in 2008 when Hillary Clinton finished substantially better than the polling had been predicting she would. In 2016, many polling companies have responded to these difficulties by simply not polling Nevada much at all, so we’ve got a limited amount of data to work with headed into today’s voting.

RealClearPolitics, for example, is only listing three polls since the start of 2016 that it apparently considers credible enough to be included in the average it calculates. One, from a firm called Targetpoint that was conducted for the conservative publication the Washington Free Beacon shows a tie between Sanders and Clinton, which is a dramatic shift from the twenty point lead Clinton had in the limited amount of polling done in 2014. A second poll from CNN/ORC shows Clinton with a one point lead, and a third from Gravis Marketing gives Clinton a six point lead. All of this adds up to a 2.4 point advantage for Clinton in the RealClearPolitics average, and a similar number from Pollster since they are looking at the same limited universe of polling.

This is hardly a sufficient universe of polls from which to make a real judgment about where the race stands at this point. By the end of tonight, or tomorrow morning, or whenever the results from Nevada can be expected, we could end up seeing Clinton pull off a solid victory, or a very narrow one, or we could see a narrow Sanders victory. The prospect of Sanders blowing Clinton away here as he did in New Hampshire seems unlikely, but it is certainly not impossible. In the end, if I had to guess I would project that Clinton will end up winning in Nevada, most likely by a narrow margin of no more than five percentage points. From there, the Democrats will head to their own South Carolina next weekend, where Hillary Clinton’s firewall seems to be holding.

Update: Jon Ralston, perhaps the one person who understands Nevada politics best, explains the Democratic Caucus process. Based on his estimates, we could see results from Nevada around the same time polls close in South Carolina.