(CNN) Health care benefits are really important to many union workers -- important enough to give up pay raises or even to walk off the job to keep the coverage they've negotiated.

Now the future of those benefits is at the heart of an emerging split among 2020 Democratic candidates over how to remake American health care. Former Vice President Joe Biden says union members shouldn't have to give up their employer plans if they like them, while Sen. Bernie Sanders is arguing that union workers would still come out ahead under "Medicare for All," which would shift all Americans into a government-run plan.

The fight puts Sanders, a declared democratic socialist, at odds with some in the labor movement, where many union workers already have insurance with pretty good benefits. Sen. Elizabeth Warren, who is also seeking the nomination, is in a similar situation because of her support for both Medicare for All and organized labor.

Already, there's a growing divide among unions as to whether to back Sanders' universal coverage proposal. National Nurses United, the largest union of registered nurses, are very vocal supporters of Medicare for All. But the International Association of Fire Fighters opposes eliminating employer-based coverage

Still other unions, such as the Service Employees International Union, say they support all efforts to expand coverage to more people.

Roughly 10.5% of workers -- or 14.7 million people -- were union members in 2018, according to the Bureau of Labor Statistics. That's about half the share that were unionized 35 years earlier.

Some 95% of union workers had access to employer medical benefits in 2019, compared to 68% of non-union workers, according to the bureau's data. And they were more likely to participate in the plans, with 84% of union staffers signing up, compared to 54% of their non-union counterparts.

"Bargaining for health benefits is a distinguishing characteristic that labor unions hold out as an important variable as to why you would want to be a union member," said David Brenner, national director of multi-employer consulting at Segal Consulting, a leader in the field.

The quality of their health plans varies widely, experts said. Unions that cover a lot of lower-wage employees, such as custodians or nurse's aides, may have more basic benefits and have to pay more for them. But other unions, including those in the construction, auto industry and some hotel trades, have more generous policies -- with low premiums, deductibles and co-pays. A few have even set up their own health clinics for members to get care.

Many health packages are so comprehensive that unions feared getting hit with the Affordable Care Act's Cadillac tax, which would have slapped a levy on the value of employer-provided coverage above a certain threshold. Companies and unions quickly banded together to fight it, fearing they'd have to curtail workers' health benefits. The implementation date has been delayed twice, and the House recently voted to repeal the provision

The United Auto Workers, who are currently striking against General Motors, are among the unions with high-quality plans. Members pay only about 3% of the total cost of their health care, while salaried staffers have to cover about one-third, according to GM.

Until recently, the automaker had been seeking to shift more of the bill to the UAW workforce. But GM has dropped its request for major changes in the health care plan. Talks are now hung up on other issues, including pay, the use of temporary workers and a union demand to shift work from Mexico to facilities slated to close in the US.

However, GM has also stopped paying for health insurance for the striking workers, who will have to sign up for COBRA. The UAW's strike fund is picking up the cost. Sanders took the opportunity earlier this week to note that union members wouldn't have to fear losing their benefits during strikes under Medicare for All.

Medical benefits is a central component of most contract negotiations. Health care is a major expense for most companies, and they have been pushing more of the cost onto non-union employees for years. They want organized labor to contribute more too.

But union workers are willing to make sacrifices to shield their health care coverage, including curtailing raises or other benefits.

"In an awful lot of cases, workers are forgoing any increase in wages in order to keep their deductibles from going even higher," said Tom Leibfried, legislative representative for the AFL-CIO, a federation of 55 national and international labor unions that represent 12.5 million people.

There's also a growing trend of contract talks breaking down in part over health benefits, said Alex Colvin, dean of Cornell University's ILR School, formerly known as the School of Industrial and Labor Relations. Health coverage was among the issues in United Steelworkers' strike against National Grid in Massachusetts and the teachers' strike in West Virginia last year.

"There's a lot more labor unrest and the cost of health care is one of the drivers of that," Brenner said.

Health care is important to union workers for several reasons, said Shaun Richman, program director at the SUNY Empire State College's School of Labor Studies who also was a deputy director of organizing for the American Federation of Teachers.

"You can take a higher raise to the bank, but when your kid gets sick, the peace of mind of good health insurance is important," Richman said. Also, "it's a badge of honor of maintaining the health insurance and not taking concessions on it."

Recognizing that he needs unions' support, Sanders issued a Workplace Democracy plan last month that would require companies to pass along savings resulting 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, as long as it doesn't duplicate the benefits available under Medicare for All.

A Warren aide told CNN recently that the campaign is talking with labor leaders about how unions with negotiated health care plans will be guaranteed a chance to rework their agreements during a Medicare for All transition.

That may not be enough for some unions, though.

"There has to be a role for the hard-fought, high-quality plans that we've negotiated," AFL-CIO President Richard Trumka told Fox News earlier this month. "If there isn't some way for us to have our plans integrated into the system, then we would have a hard time supporting it."