Charging that Congress is violating the only law it ever specifically promised to obey, a business-backed public interest law firm is preparing to take the House and Senate to court today for violating the Clean Air Act.

It's not smoke-filled rooms that are the problem, it's the Capitol Power Plant, a cinder in the eye of Capitol Hill residents for years.

The Capital Legal Foundation plans to sue the Congress, the architect of the Capitol and everyone else responsible for the plant, including House Speaker Thomas P. O'Neill Jr. (D-Mass.) and Senate Rules Committee Chairman Claiborne Pell (D-R.I.).

The foundation, which draws its support primarily from business organizations and excutives, served formal notice of its intent to go to court over the polluting plant last May and today plans to file a lawsuit in U.S. District Court for the District of Columbia, said James Richards, legal director of the foundation.

The lawsuit will ask the courts to order Congress to install the latest in pollution control equipment on the Capitol Power Plant, which Richards says hasn't added any new emission control gear for a decade, despite constant complaints about pollution.

"I'm trying to reach Congress an object lession," Richards said. Noting that the U.S. government usually is exempt from obeying federal laws, Richards said the Clean Air Act is "the first and only law Congress has made itself subject to, and they're violating it."

Located at New Jersey Avenue and E Street SE, the Capitol Power Plant heats and cools the Capitol, the Supreme Court -- where the case could end up -- and several other federal buildings.

Last December and again in May, officials of the D.C. Environmental Health Administration complained that the plant was pouring out visible smoke, even apparently at night in hopes that the pollution wouldn't be noticed.

Documents obtained by the foundation in preparing its lawsuit also show that on at least one occasion, officials at the power plant refused to accept a violation notice from a city inspector who caught the plant breaking the law.

"Any businessman who refused to accept a notice of violation would find himself hauled into court and facing fines of $10,000 a day," Richards complained. "But the Capitol Power Plant can get away with it because the plant belongs to Congress."

The District of Columbia has never taken Congress to court, even though local inspectors have spotted illegal smoke coming from the plant on several occasions. In February 1978 and again last December, D.C. pollution inspectators issued formal notices of illegal pollution at the plant, according to documents obtained by the foundation.

But in May -- after the foundation threatened to sue -- John Brink, chief of the District's Bureau of Air and Water Quality, notified city lawyers there was no basis for presecuting the plant's owners.

"Our personnel have observed, during the last winter, emissions which might have been violation," Brink conceded in a letter to the District's corporation counsel."During these instances however, the persons invovled did not have an opportunity to take proper readings and thus were unable to document any violation."

Bailus Walker Jr., the Environmental Health Administration official who signed the violation notices sent to the plant last winter, insisted Brink's letter did not mean the city was afraid to take on Congress, which approves the District's budget.