He lacks the swank offices and six-figure salary Washington lawyers often command, but in a city captivated by influence and impact, Alan B. Morrison is clearly in the major leagues.

Morrison, the director of the Public Citizen Litigation Group, is leading the constitutional challenge of a group of House members to the controversial Gramm-Rudman-Hollings balanced-budget law that Congress enacted last year.

The battle over the law and the constitutional questions Morrison has raised about separation of powers are likely to dominate congressional debate for months, until the Supreme Court has a chance to consider the case and rule on it, probably by summer.

It is not the first time that Morrison, considered one of the sharpest, most articulate courtroom debaters in town, has been center stage in a major court test of a constitutional issue.

Director of the nonprofit litigation group since it was set up by Ralph Nader 14 years ago, Morrison has successfully taken on federal government agencies over the Freedom of Information Act; the Nixon administration over the firing of Watergate Special Prosecutor Archibald Cox and the impounding of funds already approved by Congress; the legal community for setting fixed fee schedules, and Congress for using the legislative veto to overrule executive branch actions it didn't like.

His 1983 Supreme Court victory in the legislative veto battle is now widely viewed as one of the most significant separation-of-powers rulings by the court. It threw into question more than 200 laws passed by Congress, including the War Powers Act, and has permanently changed the way business is done on Capitol Hill.

"He's genuinely concerned about making the government work. He believes in the legal system . . . that it can be made to work for everyone. He's not just picking the issues that cut in favor of his particular constituency. In fact, it's often unclear what his particular constituency is," said Georgetown associate law professor Gerry Spann, who once worked at the litigation group with Morrison.

While Morrison is often characterized as a liberal Democrat, he said that concerning the Constitution he is a "conservative," a firm believer that the system of checks and balances at the heart of constitutional government protects everyone.

"I like to think I'm on the side of the Constitution," said Morrison, of his frequent appearances in separation of powers cases. "I don't have a vendetta against Congress or the executive branch. I just think these shortcuts are destructive of our democracy."

In U.S. District Court last week Morrison argued that the Gramm-Rudman-Hollings law illegally puts the budget process on "autopilot" because it turns over spending decisions to unelected bureaucrats. It amounts, he said, to a dangerous abdication of lawmaking power by the legislative branch.

The budget-balancing law -- named after its chief sponsors, Sens. Phil Gramm (R-Tex.), Warren B. Rudman (R-N.H.) and Ernest F. Hollings (D-S.C.) -- mandates a balanced federal budget by fiscal 1991. If Congress fails to meet annual deficit targets set in the law to meet the goal of a balanced budget, the law requires automatic across-the-board spending cuts.

Those automatic cuts would be based on deficit calculations made by three agencies -- the Congressional Budget Office, the Office of Management and Budget and the General Accounting Office -- and then carried out by a presidential order.

"What bothers me about that thing [Gramm-Rudman] is the total lack of accountability," Morrison said, adding that it was clear when the politically popular law whipped through Congress, "nobody cared what the constitutional issues were. That wasn't what anybody was thinking about or caring about."

As a result he quickly jumped at the chance to challenge the law when approached to do so by Rep. Mike Synar (D-Okla.), first helping Synar fashion amendments to Gramm-Rudman to ensure a quick court test and then agreeing to argue the case once Gramm-Rudman became law.

It is this sort of legal contest, Morrison said, that more than compensates him for the low pay and high workload of public-interest law. A graduate of Yale College and Harvard Law School, Morrison was two years into a career in a Wall Street law firm in 1968 when he gave up the often gray world of corporate law for the excitement of litigation. "I would go absolutely crazy if I was carrying somebody's briefcase around," he said.

He did a four-year stint with the U.S. attorney in the Southern District of New York, leaving there to head up the litigation group.

Except for stints teaching at Harvard -- including this month while he was preparing to argue the Gramm-Rudman case -- Morrison has been with the litigation group ever since. Today he is paid a little over $40,000 a year, an amount he acknowledges is well below what many of his law students will make in their first year out of Harvard.

But few of his law students are certain to claim the same satisfaction that Morrison takes in his work.

In 14 years he and the 10-lawyer litigation group have argued 15 cases before the Supreme Court. Unlike many lawyers in town, he is not just "a hired lip" going to court on issues he cares little about. What he chooses to take on frequently changes the way government works. And that is not only intellectually satisfying, Morrison said, it is also "a lot of fun.