Johnson, of Wisconsin, wrote in the Wall Street Journal that the Obama administration exceeded its legal authority by arranging federal subsidies for members of Congress and their staff under the Affordable Care Act, known as Obamacare.

"The president and his congressional supporters have also broken their promise to the American people that Obamacare was going to be so good that they would participate in it just like everyone else," Johnson wrote. "In truth, many members of Congress feel entitled to an exemption from the harsh realities of the law they helped jam down Americans' throats in 2010."

Unlike millions of Americans, he wrote, lawmakers and their staffs can receive employer contributions to help pay for their health insurance.

Johnson said the lawsuit, to be filed in the U.S. District Court for the Eastern District of Wisconsin, would challenge an October ruling by the Office of Personnel Management "that ignores the clear intent and language of the law."

Lawmakers and their staffs received an exemption allowing them to be covered under a Small Business Health Options Program, he said. The program allows the employer - the federal government - to buy health insurance for employees.

"After groping for a pretext, OPM essentially declared the federal government a small employer," Johnson wrote. "Neat trick, huh?"

The ruling applied to the 535 members of the Senate and House of Representatives and thousands of Capitol Hill staff.

Johnson said the OPM decision forces him and other lawmakers to participate in an activity they believe is illegal.

"It also potentially alienates members of Congress from their constituents, since those constituents are witnessing members of Congress blatantly giving themselves and their staff special treatment," he wrote.

Obamacare offers subsidized private coverage to lower-income families through new state marketplaces, expands government insurance for the poor and adds consumer safeguards and cost-saving initiatives for the healthcare industry.

The law has spawned many lawsuits, most recently from a group of Roman Catholic nuns over the part of the law requiring employers to provide insurance policies covering contraception.

When Congress passed the health reform law in 2010, an amendment required that lawmakers and their staff members purchase health insurance through the online exchanges the law created. They would lose generous coverage under the Federal Employees Health Benefits Program.

The author of the amendment, Republican Senator Charles Grassley, argued that if Obamacare plans were good enough for the American public, they were good enough for Congress. Democrats, eager to pass the reforms, went along with it.

But it soon became apparent the provision contained no language that allowed federal contributions toward their health plans. This caused fears that staff would suddenly face sharply higher healthcare costs and leave federal service.

The OPM said the federal contributions will be allowed to continue for exchange-purchased plans for lawmakers and their staffs, ensuring that those working on Capitol Hill will effectively get the same health contributions as millions of other federal workers who keep their current plan.