Through years of acrimony over Obamacare coverage for the poor and other individuals lacking health policies, one kind of insurance has remained steady, widespread and relatively affordable.

Employer-sponsored medical plans still cover more Americans than any other type, typically with greater benefits and lower out-of-pocket expense. Recent cost increases for job-based coverage have been a tiny fraction of those for Obamacare plans for individuals.

Now, as President Donald Trump promises a replacement for the Affordable Care Act that will provide "insurance for everybody," employers worry Republican attempts to redo other parts of the insurance market could harm their much larger one.

"We're deeply embedded" in the health law, said Neil Trautwein, vice president of health care policy for the National Retail Federation, a trade group. "Pick your analogy — it's like being tied to the railroad tracks or having a bomb strapped across your chest. It's tough to disarm these things."

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Business dislikes many parts of the ACA, including its substantial paperwork, the mandate to offer coverage and the "Cadillac tax" on high-benefit plans that takes effect in 2020. But large companies in particular — those that have always offered job-based insurance — say a poorly thought-out replacement might turn out to be worse for them and their workers.

"Whatever the Republicans are going to do, they've got to make it look as different from the ACA as they can" for political reasons, said Edward Fensholt, a senior benefits lawyer at Lockton Companies, a large broker. "There are some pieces that aren't broken, and the more you ... make something different from the ACA, the more you risk screwing up things that look OK."

Any new health law needs substantial revenue to replace the Cadillac tax as well as ACA taxes on health insurers, medical devices and high-income households that paid for care expansion — assuming those measures are repealed. Otherwise, it risks stranding the millions getting government-subsidized Obamacare coverage.

One tempting solution for Republicans, big business worries, is to limit the exemption from income and payroll taxes that job-based coverage has enjoyed for decades. Taxing workers and employers for health benefits could raise billions to pay for a replacement plan.

Such a measure has occasionally been floated by Republicans since the days of President Ronald Reagan, often under the argument of leveling the field between employer plans and other coverage that the tax code treats far less generously.

Republicans are far from consensus on what a replacement should look like.

But capping the tax exclusion for employer plans is in replacement legislation introduced by Rep. Tom Price, the Georgia Republican poised to become secretary of Health and Human Services. It's also in House Speaker Paul Ryan's Better Way plan. A third proposal would keep the ACA taxes.

Related: What Obamacare could be replaced with under Trump

Depending on how it's structured, limiting the employer-plan loophole could add hundreds of dollars to workers' tax bills as a portion of their expensive health insurance is counted as taxable income.

"I can't understand why nobody has asked the president about this," said James Gelfand, the top health policy official at ERIC, a lobbying group of very large employers focused on benefits and pay. Taxing employer insurance to subsidize non-employer insurance, he said, "is a 100 percent redistribution of wealth."

The argument is a self-interested one for big business. A tax on health benefits would be a pay cut for its workers. Corporate payroll taxes would also rise if the value of worker health benefits counted toward required employer contributions to Social Security and Medicare.

But independent experts share their concern.

Taxing medical benefits could spur employers to erode or drop them and make workers less likely to accept them, said Larry Levitt, a senior vice president with the Kaiser Family Foundation. (Kaiser Health News is an editorially independent program of the foundation.)

More concerning, he said, are proposals such as Price's that could give workers on company plans the option to buy outside individual coverage using tax credits. While offering workers more choice — a Republican goal — it would also erode the ACA's "firewall" protecting employer coverage from market turbulence.

"There could be much more movement from employer coverage to individual plans, especially among younger and healthier workers," Levitt said.

That could leave company plans with the same "adverse selection" problem experienced by many individual plans sold via Obamacare's online marketplaces — too high a portion of older, sicker, more expensive members, Fensholt said.

Related: Insurers to Congress: Stabilize Obamacare in next 30 days

"What the administration and Congress must do immediately is stabilize the individual insurance marketplace and not destabilize the employer marketplace," said James Klein, president of the American Benefits Council, which advocates for large companies.

Job-based coverage has remained stable even as annual premium increases reached double-digit percentages and enrollments missed expectations for individual plans, pressuring the market despite the support of government subsidies.

Employer health plans cover some 150 million people, while last year individual plans covered about 20 million, according to Mark Farrah Associates, a data firm. That included some 10 million who bought coverage through the health law's subsidized exchanges.

The portion of non-elderly Americans covered by employer-sponsored plans has stayed almost unchanged since the health law was passed.

Recent premium increases for job-based plans have been below 5% on average, less than the historical trend and far lower on average than premium hikes for individual insurance. Employers still pay most of the premiums.

One way companies have contained premiums is by shifting to high-deductible plans in which the insurance covers little until workers have incurred thousands in medical bills. Even so, benefits in employer plans are generally richer than those in the average individual plan, experts say.

Related: Even as they repeal Obamacare, Republicans still have to pay for it

In the contentious political climate, executives at individual companies were reluctant to be interviewed for this story. More than a half dozen employers declined to comment.

But there is enormous interest and concern among benefits executives in what a Republican plan might look like. A post-election session on repeal and replacement drew the biggest webinar audience ever for the National Business Group on Health, said Steve Wojcik, the advocacy and research group's vice president of public policy.

If the audience was anything like the experts at the American Staffing Association, it had some catching up to do.

"We ignored lots of the talk of the Republicans about repeal," said Edward Lenz, senior counsel at the group, which advocates for temp and recruiting firms. "First of all there was no chance of that happening with the president [Obama] in the White House. And frankly no one here counted on President Trump's victory."

Kaiser Health News is national health policy news service that is part of the nonpartisan Henry J. Kaiser Family Foundation.