As support grows on Capitol Hill for taxing workers’ medical benefits to pay for a health care overhaul, Democrats on the House’s tax-writing panel hold a thin line of resistance.

Although they have backed away from the complete opposition Chairman Charles B. Rangel voiced earlier this year, Ways and Means Committee members remain reluctant to alter the longstanding exclusion of health benefits from taxable income.

Many committee Democrats are unwilling to broach the subject directly. Instead, they say they want to work out how to expand health coverage and see how much money they can raise through promoting efficiency in the health care system before discussing the revenue side of the bill.

“We’ve got to design a public option first,” said Jim McDermott , D-Wash., referring to a government-run insurance plan that would compete with private companies. “Then we’ll figure out how to pay for it.”

But Senate Finance Committee members, who have spent more time working through the details of a health bill, have generally abandoned that approach. Cost savings alone, they argue, are too uncertain and cannot generate enough money to pay for covering all the uninsured. They would also prefer to raise money for health care from within the health sector.

Finance Chairman Max Baucus , D‑Mont., said Thursday it would be “difficult” to move a health care bill that does not alter the tax exclusion. “It’s got to be done in a very sensitive way, to make sure the limits are high enough,” he said, adding, “I think some version is a real possibility, hopefully an appropriate version.”

Numerous Critics, Staunch Defenders

The exclusion allows employees to receive unlimited untaxed compensation in the form of health benefits. Liberal critics call it regressive, because the benefits of the exclusion tend to flow to higher-income workers. Others argue that it encourages overspending on health care because workers and companies don’t bear the full cost of medical decisions. And others say it creates a disparity between people who get health insurance through their employers and those who purchase it on their own with after-tax money.

“We’re trying to restructure our health system in a way that provides subsidies that are more directed on those who really need it, and also we’re trying to create a system that, at the end of the day, delivers health care more efficiently,” said Paul Van de Water, senior fellow at the Center on Budget and Policy Priorities, a liberal think tank. “Reducing the tax preference for employer-paid insurance moves toward both of those goals.”

The current full exclusion reduces potential revenue by about $250 billion per year, meaning that a cap on the tax break could make a significant dent in the cost of a health care overhaul expected to cost about $1 trillion over 10 years.

But changing the exclusion would be politically challenging. Unions with generous health care plans oppose any change to the exclusion, and President Obama spent his campaign last year shredding opponent John McCain ’s proposal to repeal the exclusion and replace it with a tax credit for purchasing health care.

Despite occasional hints of openness to changing that position, the White House has consistently pointed to the campaign trail statements as the president’s position on the issue.

Obama proposes to pay for part of the health care bill by capping itemized deductions, including those for charitable deductions and home mortgage interest, at 28 percent. That has fallen flat in Congress so far, which turned attention again to the exclusion.