(CNN) Sen. Bernie Sanders has pitched himself as the most ambitiously pro-labor candidate in the Democratic primary. He has also centered his campaign on the promise of passing and implementing a "Medicare for All" single-payer health insurance system.

But in Nevada, where unions remain key power brokers in Democratic politics, those priorities are coming into stark conflict -- and heightening divisions in the labor movement.

Unions have been one of the most politically potent sources of opposition to Medicare for All throughout the 2020 Democratic campaign. The Vermont senator's rivals have repeatedly seized on labor's anxieties over a fundamental reordering of the country's health care system. That skepticism largely stems from worries over what the transition would mean for the generous benefits their unions negotiated, often over decades and at the expense of pay raises, with employers.

In the face of those concerns, Sanders and sympathetic labor leaders have cast Medicare for All as a long-term winner for union workforces, arguing that any legislation that takes health care out of the bargaining process will ultimately lead to higher wages and new concessions from management.

He reiterated his support for unions at Wednesday's debate: "I have a 30-year 100% pro-union voting record."

The bust-up over health care has been most sharply fought over the last week in Nevada, home to the largest union presence of the early voting states. Some 14.6% of workers in the Silver State were union members in 2019, compared with 6.3% in Iowa and 10.3% in New Hampshire, according to the Bureau of Labor Statistics.

Both in Nevada and around the country, the fight has opened up rifts in the labor movement. Some unions, such as National Nurses United and the American Postal Workers Union, are vocal Sanders supporters. He also has the backing of Nevada's largest teachers union, the Clark County Education Association. SEIU Local 1107, another massive in-state union, has not endorsed any candidate, but pushed back this week against criticism of Medicare for All.

Other major unions, including the International Association of Fire Fighters, an early endorser of former Vice President Joe Biden, and the uncommitted AFL-CIO, have made their doubts clear. The leadership of Nevada's influential Culinary Union declined to support a candidate in the primary, but in making that announcement, and through its distributed literature, has signaled to members that it considers Medicare for All a nonstarter.

The Culinary Union's case

The Culinary Union Local 226, which is part of the labor union Unite Here, has held town halls with several presidential candidates in recent months to hear about their proposals for health care, immigration, jobs, the economy, climate change and other issues. The union has also been polling and surveying members to learn their views, said Geoconda Argüello-Kline, its secretary-treasurer.

Signs that Sanders was running into trouble with the Culinary Union surfaced earlier this month when the leaders circulated a flyer that said, without naming names, that some candidates would make union members give up their health care

"We will not hand over our healthcare for promises," it read. "We have fought for 85 years to protect our healthcare. Why would we let politicians take it away?"

Then the union distributed another flyer , which listed the positions of all the candidates and described Sanders' plan as one that would "End Culinary Healthcare" if he becomes president.

Sanders' supporters responded by "viciously" attacking the union , according to Argüello-Kline, who defended the scorecard, saying it simply presented members with the "facts" about the candidates. (Sanders, for his part, disavowed the attacks, saying at Wednesday's debate that he would not be surprised if they were carried out by Russians interfering with the election.)

Medicare for All skeptics lean toward more moderate proposals pushed by Biden, Minnesota Sen. Amy Klobuchar and former South Bend, Indiana, Mayor Pete Buttigieg, all of whom would add a government-run insurance option to the Affordable Care Act exchanges but allow unions and employers to keep their private coverage.

The Culinary Union flyer said Buttigieg, Biden, Klobuchar and billionaire Tom Steyer would "Protect Culinary Healthcare." Massachusetts Sen. Elizabeth Warren's two-part Medicare for All implementation plan got more nuanced treatment. Warren would "Replace Culinary Healthcare" after a three-year transition period or at the end of collective bargaining agreements, the flyer said.

While the Culinary Union isn't backing any single candidate, its leadership has made clear that the 60,000 workers it represents value their health insurance plans and the ability to make choices, a point Argüello-Kline repeated several times during a 25-minute news conference.

"We believe every human being, they have the right to have health care," Argüello-Kline said. "But everybody has the right to make choices."

Health care is the top issue that the union fights for in contract negotiations with employers, she told CNN, noting that members have gone on strike to shield their benefits.

In attempting to boost and consolidate labor support, Sanders has tried to soothe their concerns . He issued a Workplace Democracy plan last summer that included a requirement that companies pass along saving from Medicare for All to workers in the form of raises or other benefits. Unions would be allowed to maintain their clinics and provide supplemental coverage, according to the proposal, as long as they don't duplicate the benefits available under Medicare for All.

Still, the issue became a point of tension when Sanders met in December with Culinary Union members. A woman in the crowd asked why he wanted to take away their health care plan.

"We love our Culinary health care. We want to keep it. I don't want to change it. Why I should change it?" the woman asked, to applause, whistles and chants of "226," the local's number.

Some union members are also wary of Sanders' promise that they could keep their prized health clinic. They are concerned that, as a single paragraph in a much larger plan, it is too vague and lacking in detail. But the opposition to Medicare for All is largely rooted in questions over choice -- something Buttigieg hit on repeatedly as the frenzy over the union's flyers grew.

"I'm thinking, for example, of the culinary workers here in Nevada, who fought so hard, those strikes and those negotiations, to get excellent health care plans. Who are we to tell them that they have to give up those plans?" Buttigieg said at a town hall.

Sanders, meanwhile, sought to reassure union members that his plan would not strip them of their insurance.

"So let me be very clear to my good friends in the Culinary Workers Union, a great union," he said at Wednesday's debate. "I will never sign a bill that will reduce the health care benefits they have. We will only expand it for them, for every union in America, and for the working class of this country."

There's a reason why these workers want to protect the current system. The Culinary Union provides top-notch coverage to its members, who do not have to pay premiums or deductibles. They are responsible only for small copays for service. Plus, they can go to a union-built health center, where nearly all the care is free.

"Workers have enacted a vision of what working people want in terms of health care," said Bethany Khan, director of communications and digital strategy for the union. "We have it. It's tangible. It's real to us. It's not a wish. It's not an aspiration. And we would like to keep it that way."

Through its Culinary Health Fund, which was founded in the 1960s, the union is one of the largest health care consumers in the state. The fund provides benefits to 145,000 participant Nevadans.

The union also created a 60,000-square-foot health center in Las Vegas, which opened its doors in 2017. It provides urgent care, primary care, lab work, radiology and pediatric services, as well as dental and vision care. It also has a pharmacy. Most of the services are free -- there are copays only for the dental and vision services. Two more centers, which will have expanded services, are in the works.

The union's health benefits were top of mind for the dozen or so workers CNN spoke to last weekend. Only one said he backed Sanders.

Maria Rodriguez, who works for Park MGM, was carrying one of the flyers when she stepped out of the early voting location at the union.

"Right now, for them, they have provided us with an urgent care and that is really helpful. My kids, they get sick and we just take them over there," she said, referring to the clinic. "If we don't have Culinary, what can we do?"

Legislate or negotiate?

The backlash to the Culinary Union's fierce opposition to Medicare for All boiled over on Monday afternoon, when Service Employees International Union Local 1107 Executive Director Grace Vergara-Mactal, in a joint statement with the affiliated Western States Regional Joint Board's Maria Rivera, argued that the fight was rooted in a misguided narrative.

"The debate over whether working Americans need Medicare for All or union-negotiated health benefits is a false choice," Vergara-Mactal and Rivera said. "While some working people have good healthcare plans through their jobs, many do not. Access to quality healthcare shouldn't be based on luck, it should be a right."

SEIU Local 1107 is smaller than the Culinary Union but still represents close to 20,000 workers and, while the union has not endorsed, its membership favored Sanders in a December straw poll. Warren stopped by the local's bargaining meeting on Monday for a wide-ranging conversation that included more sympathetic questions about her version of Medicare of All.

The union's chief of staff, Brian Shepherd, told CNN that he understands "Culinary's position -- that they fought for and have sacrificed a lot to get the health care that they have," but his organization "takes a different approach."

Shepherd pointed to the makeup of its rank and file, which includes a variety of health care workers, including nurses and other hospital staff, as one potential reason it takes a contrasting view of the question. He also pointed to the weight that health care places on unions during contract talks with employers.

"Taking (health care) off the bargaining table would be helpful to our membership," Shepherd said, "because every year, every time we're in negotiations, the employers always try to use that. They sort of weaponize it against our membership and try to get them to make other sacrifices in order to keep health care, which we just don't think is right."

Other Nevada unions are making their support for Sanders clear. The Clark County Education Association is the state's largest teachers union in the country's fifth biggest school district. In January, it endorsed Sanders after taking a straw poll of members.

Kenny Belknap, an executive board member of the Clark County Education Association who teachers US government to high school seniors, said it was Sanders' "comprehensive plan for education" -- including raising teachers' starting salaries to at least $60,000, reducing class sizes and doing away with high-stakes standardized testing -- that won over the union.

But he also cited Medicare for All as a factor that largely came down in Sanders' favor, calling it "a really big point" with a number of union members.

"I'm not saying that we have bad health care for our members," Belknap said, "but even with the best health care, you're going to have copays, you're going to have deductibles. With the teachers' salaries being what they are, it would be a huge boost to all of our incomes just by having that being taken care of rather than having to pay those monthly copays and all that."