With lawmakers back in Washington after the August recess, employer groups are renewing their drive to convince Congress to make significant changes to the health care reform law, but they know they face an uphill battle.

Much of the employer lobbying effort will be focused on a Patient Protection and Affordable Care Act provision that, when it takes effect in 2018, will impose a 40% excise tax on group health plan premiums that exceed $10,200 for single coverage and $27,500 for family coverage.

Without plan design changes, eventually most group plans will be hit by the tax, numerous surveys have found.

In 2018, the first year the tax goes into effect, nearly 50% of employers expect to trigger the tax without plan design or other changes to hold down costs, according to a National Business Group on Health survey released last month.

Business groups have argued for a variety of reasons that the tax should be repealed. One reason they give is that there is no evidence to support a key congressional assumption behind the tax: that it will generate tens of billions of dollars in new tax revenue because employers cutting benefits to avoid the so-called Cadillac tax will offset the impact of those cuts by boosting employees' taxable salaries. Few experts believe that employers will increase salaries to offset benefits cuts.

That and other arguments are being heard on Capitol Hill. For example, an excise tax repeal bill, H.R. 2050, earlier introduced by Rep. Joe Courtney, D-Conn., continues to pick up bipartisan support and now has over 140 co-sponsors.

“That is a pretty sizeable group,” said Gretchen Young, senior vice president of health policy at the ERISA Industry Committee in Washington.

“There is truly bipartisan support for repeal,” added James Klein, president of the American Benefits Council in Washington.

But both Ms. Young and Mr. Klein, as well as others, acknowledge the chance of repeal is slim.

“We are realistic about the chances. The Obama administration and some members (of Congress) are opposed to repeal. We do have our work cut out for us,” Mr. Klein said.

One big challenge is convincing lawmakers that the prime rationale for the tax — that the tax will be a big revenue raiser and generate funds to help offset the government's cost in providing premium subsidies to the lower-income uninsured obtaining coverage in public exchanges — is false.

“There is that belief that too much revenue would be lost,” said Allison Klausner, a principal with Buck Consultants at Xerox in Washington.

“While there is significant momentum to repeal the tax, we recognize that the congressional agenda is crowded, with not a lot of time,” said Steve Wojcik, vice president of public policy for the NBGH in Washington.

In addition, even if Congress does repeal the excise tax, there aren't — at least not yet — enough votes to overturn a near-certain presidential veto, observers say.

Others say that while repeal of the excise tax is a long shot, lawmakers may consider changes to the excise tax. These could include linking the health care plan cost trigger to annual increases in health care costs rather than to the consumer price index, or modifying the excise tax provision to exclude employees' pretax contributions to health savings accounts and flexible spending accounts.

Potential changes to those ACA provisions “could get a thorough congressional review” in the months ahead, said Geoff Manville, a principal with Mercer L.L.C. in Washington.

On the other hand, congressional interest is lagging for another ACA change sought by employers: changing the law's provision that defines full-time employees as those working an average of at least 30 hours a week.

Employers and their congressional backers want the definition to be changed to define full-time employees as those working an average of 40 hours a week, reflecting real-world employer practices.

That definition is important, since employers not offering coverage to at least 70% of their full-time employees this year and 95% in 2016 and succeeding years will be hit with a penalty of $2,000 per full-time employee, starting next year. But congressional interest in making such a change has lagged.

“There is very limited Democratic support in the Senate. Democrats are holding the line against passage, and I doubt that President Obama would sign such a major change” to the ACA, Mr. Manville said.

On the regulatory side, employers expect the IRS to propose guidance on the excise tax early next year. Among issues that need clarification are whether employer costs in running on-site health care clinics should be included as a health plan cost for excise tax calculation purposes.

On the other hand, regulators have shown little interest in releasing rules on an ACA provision that requires employers with at least 200 employees to automatically enroll employees in a group plan in situations where employees don't choose a plan.

“I'm not detecting much regulatory interest in that one,” Mr. Manville said, referring to the automatic enrollment provision.