The rift surfaced last week, when the 60,000-member Culinary Workers Union declined to endorse any Democrat in this week’s Nevada caucuses after slamming Bernie Sanders’ health plan as a threat to the hard-won private health plans that they negotiated at the bargaining table. But the conflict extends well beyond Nevada.

On one side of the divide are more liberal unions like the American Federation of Teachers and the Service Employees International Union, which argue that leaving health benefits to the government could free unions to refocus collective bargaining on wages and working conditions. On the other side are more conservative unions like the International Association of Fire Fighters and New York’s Building & Construction Trades Council, which don’t trust the government to create a health plan as good as what their members enjoy now.

“It’s an extremely divisive issue within the labor movement,” said Steve Rosenthal, a former political director for the AFL-CIO. “Nobody’s opinions will be changed during the presidential nominating fight, and unions may well be divided over Democratic candidates until the end.”

In New York, the New York State Nurses Association and Local 1199 of the Service Employees International Union pressed hard in 2018 for a state single-payer system. But other unions, including the New York State Building & Construction Trades Council, joined forces with private health insurers to kill the bill, funding polling to show opposition to the tax increases needed to implement it and writing op-eds calling the plan a “folly” that would “send jobs and people fleeing” the state.

Now some of those same New York labor leaders are saying much the same about Sen. Elizabeth Warren and Sanders’ Medicare for All plans. Gregory Floyd, president of the Teamsters Local 237, called the policy a “disaster” and predicted that few of his 24,000 members will vote for a candidate who supports it. Floyd declined POLITICO’s request for an interview, but said his opposition to Medicare for All is “based on what is best for our members.”

In California, the aggressively pro-Sanders California Nurses Association has long pressed for state-level single-payer, to the point of circulating in 2017 an image of the state mascot, the California grizzly bear, with a knife in its back after the state Assembly leader shelved a single-payer proposal.

The union’s parent organization, National Nurses United, is deeply involved in the 2020 race — endorsing Sanders, criticizing any candidate who doesn’t embrace Medicare for All, and sending armies of members and supporters to phone banks and doorsteps in all 50 states to press for a House vote on single-payer. Earlier this month, National Nurses United announced a new campaign to pressure presidential and congressional candidates to refuse donations from a health industry lobby group that’s spending heavily to kill any possibility of single-payer — a pledge most moderate candidates are likely unwilling to take in an election marked by record fundraising and spending.

Medicare for All is notably unpopular with swing voters in the battleground states of Michigan, Minnesota, Pennsylvania and Wisconsin, according to a December poll by the Kaiser Family Foundation and Cook Political Report.

In Michigan, where 28 percent of the electorate belongs to a union, and where Sanders stunned Hillary Clinton with an upset in 2016, unions have stayed largely silent on the issue. “There is very clearly a split between union leadership and the union rank and file,” said Eli Rubin, president of Michigan for Single Payer Healthcare.

According to a poll released in July by the Detroit Regional Chamber of Commerce, a 58 percent majority of “strong Democrats” favored Medicare for All but only a 48 percent plurality of Democratic-leaning voters. Among all voters, 52 percent opposed Medicare for All. Elderly voters (who turn up at the polls disproportionate to their numbers) were especially resistant, with 59 percent opposing single-payer plans.

Reflecting the divide is Gov. Gretchen Whitmer, a centrist Democrat who opposed single-payer during her 2018 campaign but has since vaguely said she supports the idea “in concept.”

Compounding this ambivalence inside the state is labor’s ties to health care. Leaders of the AFL-CIO, the Michigan Education Association, the United Auto Workers, and Teamsters serve on the board of Blue Cross Blue Shield, the state’s largest insurance company. Whitmer’s own father, Richard Whitmer, was the longtime president of Blue Cross Blue Shield, and the company was among the top donors to her gubernatorial campaign.

Meanwhile, in Nevada, the war over Medicare for All is in full swing in Nevada ahead of the Feb. 22 caucuses. Sensing an opening after Culinary 226’s public rebuke of Sanders, many of his Democratic primary rivals swiftly and loudly sided with the union, with some (Joe Biden, Pete Buttigieg) emphasizing that they would give labor a choice of whether to keep the health plan they bargained for or switch over to a government-run public option, and with Warren promising that unions will be at the table when the details of overhauling the U.S. health system are hammered out.

But supporters of Medicare for All have successfully persuaded some unions to back the policy, or at least remain neutral. When Sanders and Rep. Pramila Jayapal (D-Wash.) rolled out revamped versions of their single-payer bills in 2018, they did so with the official backing of the Service Employees International Union, the American Federation of Teachers, National Nurses United, the American Federation of Government Employees and others.

In an interview, Jayapal said her main argument to unions is this: Even if they fear the unknown, the current system is unsustainable.

“Look, I respect where they’re coming from,” Jayapal, the lead author of the House Medicare for All bill and the health policy chair of Sanders’ campaign. “They bargained hard and gave up wages for these health care benefits and they’re worried. But health care costs continuing to rise is a certainty. And when that happens, wages are going to decline.”

Local unions, which tend to be more outspoken than their national counterparts, are playing an outsize role in the 2020 race. That’s because so many national unions have thus far held back or pledged to remain neutral in the primary. It’s a backlash from 2016, when several big unions endorsed Hillary Clinton early on, only to witness a revolt from their rank-and-file members who supported Sanders.

With locals’ growing influence is a tendency for organized labor to balkanize its support. For example, the independent group Labor for Bernie said Tuesday that more than 1,200 members of the International Brotherhood of Electrical Workers have signed a petition calling on the national union to retract its endorsement for Biden.

“I don’t know where these people are coming from,” said Rand Wilson, a co-founder of the independent group Labor for Bernie and an organizer for SEIU Local 888 in Massachusetts. “Do they go to the negotiating table? Because they’re on a different planet than me.”

But Nelson, who represents more than 50,000 flight attendants across the country, says Medicare for All supporters are only hurting their own cause when they criticize labor groups that aren’t yet on board.

“If you are not approaching this as an organizer and building a supermajority for this change, it’s not going to happen,” she said. “You have to open your arms wide and give space for everyone to share their concerns and ask questions, and you provide information and find common ground. You don’t shut down conversations.”

Jeremy B. White contributed to this report.