When President Reagan fills two vacancies on the U.S. Court of Appeals for the District of Columbia, an action expected in the next few months, he will end liberal domination of what was once the most progressive court in the nation.

For more than two decades the court stood at the cutting edge of judicial activism: It championed the rights of the mentally ill; it upheld the rights of tenants to habitable conditions; it broadened the rights of criminal defendants; it allowed consumer and environmental groups to successfully challenge federal agencies.

But today the court's jurisdiction over criminal matters -- the starting point for many of its seminal decisions -- is sharply curtailed. And where once it found kindred views in Earl Warren's Supreme Court, the Supreme Court under Warren E. Burger has battered the appeals court with an endless series of reversals.

Reagan's appointees will replace one judge who retired and fill a 12th seat on the bench that was recently created by Congress. The appointments will result in a formal 6-to-6 split between conservatives and liberals on the appeals court, and the house that former chief judge David L. Bazelon built, which has been crumbling for years, will be no more.

"The days of a predictable [liberal] result are over," said Jonathan Rose, former assistant attorney general and member of the Reagan administration judicial selection committee.

Liberals agree. "We're going to have to be much more cautious about litigating in the District of Columbia," said American Civil Liberties Union legal director Arthur Spitzer.

The appeals court, sitting in the lap of the federal government, is considered the country's second most powerful court. It rules more than any other appellate court on the constitutionality of federal laws and agency regulations and often resolves disputes between the executive and legislative branches.

When President Truman put Bazelon on the court in 1949, the court had even broader powers. In addition to being the most important of the nation's then-11 federal appeals courts it was also, in effect, the supreme court for the nation's capital. It set the law for the District of Columbia.

Bazelon, later joined by Judge J. Skelly Wright, in a series of rulings in seemingly run-of-the-mill criminal cases, greatly expanded the rights of criminal defendants and particularly rights of the mentally ill.

One of his most famous was the 1954 case of Monte Wayne Durham, a man convicted of stealing a suit from the Georgetown home of Alger Hiss' brother. The general test in criminal insanity cases had been whether the defendant could tell right from wrong.

Bazelon said that a person -- even if he knew what he was doing was wrong -- could not be judged criminally responsible if he had a mental disease or defect at the time of the crime and the crime was a product of that. The controversial ruling, called the Durham Rule, was adopted by courts throughout the country.

Throughout the 1960s, the court was narrowly and often bitterly divided between liberals and conservatives. Bazelon headed the five-judge liberal majority.

His arch-rival, then-Judge Warren E. Burger, headed the four-member conservative minority. In 1962, when Bazelon became chief judge of the court, eligible to serve until 1979, Burger reportedly fumed at "another 17 years of this." The relationship between the two was, at best, icy.

But it was Bazelon who held sway, in large part because he had the backing of a majority of the Supreme Court under Chief Justice Earl Warren. That backing meant the Supreme Court would generally decline to review the appeals court's decisions or, if the Supreme Court heard the appeal, it would generally affirm the Bazelon court opinion, applying the appeals court's views nationwide.

Conservatives in Congress were furious when the appeals court came out with decisions restricting police power, but there was little they could do.

Then, in 1970, they got their revenge by helping to push through the D.C. Court Reform Act.

That law, hailed as a progressive step toward home rule in the District, also stripped the federal appeals court's power to review the overwhelming majority of local criminal cases. Appeals from the D.C. Superior Court from that point on would go to another court -- the local D.C. Court of Appeals -- and then directly to the Supreme Court.

As a result, the appeals court's criminal cases dropped from 440 in 1970 to 111 in 1980 and to 62 last year.

Still, the court reform act did not put the liberals out of business.

The reduction in criminal cases virtually coincided with a vast increase in administrative cases as the regulatory explosion of the '60s and '70s reached the court. In 1970 the number of administrative cases filed in the court was 180, and by 1981 the number had reached 849. Last year there were 563 such cases filed.

A liberal Congress, looking for a politically sympathetic judicial forum, gave the appeals court exclusive or concurrent jurisdiction in several broad areas involving consumers and environmental laws. For example, even if those affected by federal environmental regulations live in Montana, they must come to the District to challenge them.

The liberal appeals court thus continued to issue even more nationally significant rulings, ranging from what it would cost to fuel homes, transport goods, buy drugs or how much effort should go into cleaning up the country's air and water and who should pay.

The court demanded the government go through procedural hoop after hoop to make sure its actions were reasonable and necessary under what was called the "hard look doctrine."

"We used to mention 'the deference owed the agencies' in every opinion," one former Bazelon clerk said recently, "but it was a joke. Every agency decision was reviewed from the bottom up. No [agency] decision was final until it had" the appeals court's approval. That approval was invariably good enough -- the Supreme Court rarely intervened.

That support from the high court began to unravel in 1969, when Burger replaced Earl Warren as chief justice. When he heard the news, Bazelon had every reason to be, as he remarked to friends at the time, "speechless and sick for a week."

Burger's first three years on the high court saw little change as the liberal Supreme Court majority still held. But the arrival of justices Lewis F. Powell Jr. and William H. Rehnquist in 1972 changed the situation dramatically.

Bazelon went from being the fair-haired boy to being the whipping boy for the Supreme Court.

The high court, which had consistently reviewed a smaller percentage of appeals from the District than from the other 11 appeals courts, in 1973 reviewed 16.2 percent of the appeals from the District and only 5.5 percent of the appeals from other circuits.

Also, the high court reversed nine D.C. circuit cases that year, more cases than it had reversed in the preceding six years.

For years, conservatives would not bother appealing to the Warren Court for fear that they would make a more limited ruling nationwide. Now they regularly appeal to the Burger court and invariably win.

In the last 10 years under Warren, the Supreme Court reversed only 12 of the appeals court's decisions in civil cases involving the federal government. In the last 15 years, the high court under Burger reversed the appeals court 74 times in such cases.

Now, liberals often decide not to appeal losing cases for fear of creating worse precedents in the Supreme Court.

Federal appeals courts conduct their business in randomly selected three-judge panels. Those who lose their cases at this level can appeal to the full court or directly to the Supreme Court.

"In important cases our odds before the full court will be considerably less than they used to be," Spitzer said. There is still a good chance that a given panel will be sympathetic, the ACLU's Spitzer said, but "the government won't hesitate to appeal to the full court."

"We will have to think hard at the beginning of a case about whether to file it and whether we would be distressed to see it go up on appeal," he said. "The time to think is before you file a case," he said, "and we're becoming more conscious of that."

Through the 1970s, as the Supreme Court was becoming more conservative, the appeals court was becoming markedly more liberal. Bazelon assumed semiretired senior status in 1979, and one moderate-to-liberal member, Harold Leventhal, died that year. President Carter appointed their replacements and two additional judges as the court expanded from nine to 11 judges.

Carter's four appointees -- liberals Patricia M. Wald, Abner Mikva and Harry Edwards and moderate Ruth Bader Ginsburg -- often gave the liberals a 7-to-4 edge for a few years.

An exasperated Supreme Court, after years of reversing the appeals court on administrative appeals, issued in 1978 what most observers consider a "watershed" opinion in the battle in a case called Vermont Yankee Nuclear Power Plant v. Natural Resources Defense Council.

It was an opinion that one lawyer several years ago called the "most venomous anyone could remember."

Environmentalists had sued the Nuclear Regulatory Agency, claiming it had not adequately considered environmental factors before licensing the Vermont Yankee nuclear power plant.

Bazelon wrote an opinion agreeing with them. A unanimous Supreme Court, including liberals Brennan and Marshall, called the Bazelon ruling "judicial intervention run riot." The opinion, written by conservative Justice William H. Rehnquist, excoriated Bazelon for "Monday-morning quarterbacking" of administrative agencies.

Lower courts were not to "stray beyond the judicial province . . . to impose upon the agency" their own "notion of which procedures are best or most likely to further some vague, undefined public good."

Despite consistent reversals, the appeals court since 1978 has continued its march, decidedly out of step with the Supreme Court.

In a study to be published in the New York University Law Review, editor Roy McLeese reviewed the Supreme Court's handling of appeals from the D.C. Circuit from 1980 to 1983.

The Supreme Court reversed the other appeals courts 60.8 percent of the time, McLeese found. But it reversed the D.C. court 89.6 percent of the time, the highest reversal rate of all 12 appeals courts.

McLeese also found that individuals and public interest groups suing the federal government invariably won in the appeals court, while the federal government prevailed when cases went to the Supreme Court.

If constant overruling has not bludgeoned the D.C. court's liberals into ideological conformity, the reversals have at least clipped the majority's wings.

"The days of a Bazelon who could expound doctrine are over," one of the court's judges said recently. The judge said that most of the court's liberal judges are generally "writing in a narrow way" to avoid reversal.

It is the conservatives who are writing the far-reaching majorities. "They have the power to expound broader legal doctrines and not be overruled," the judge said.

"They," for the moment, are led by two judges -- former solicitor general Robert H. Bork and former assistant attorney general Antonin Scalia -- both considered near the top of a short list of potential nominees in the event of Supreme Court vacancies.

Both judges come most recently from years as law school professors, Scalia from Chicago and Bork from Yale. They share ideologies that are almost diametrically opposed to the prevailing view.

Where the liberals maintain that the judiciary must stand watch over the bureaucracy and government to see that the interests of the poor or the elderly or the environment are protected, Bork and Scalia believe that this is not the job of the unelected judiciary, that Congress is charged with that responsibility.

Where the liberals opened the doors of the courthouse to public interest groups and others to challenge agencies, the conservatives can be expected to be equally diligent in closing them, limiting who can sue the government, who in the government can be sued and the scope of any judicial review.

And where the liberals have stopped writing broad opinions to influence the high court and public opinion, the conservatives, notably Bork, Scalia and more recent arrival Kenneth W. Starr, are doing just that, confident of a receptive audience just a few hundred yards up Constitution Avenue.

An example is Bork's opinion last summer that homosexuals do not have a constitutional right to privacy, and therefore the military is free to kick homosexuals out of the armed forces. Bork went on at some length to criticize the Supreme Court's decisions regarding a constitutional right to privacy.

That opinion, one liberal judge said, was an example of "supreme judicial activism . . . reaching out or focusing on an issue no one [on either side] raised."

But the liberals could only muster four votes in an effort to have the full court review Bork's decision. Ginsburg and Wright refused to go along.

The prospect of the end of liberal control heartens those who for years were on the losing end.

In cases involving, for example, occupational health and safety, "labor would race to have the D.C. Circuit" hear a case while "business would race for the Fifth [Circuit]," said Stephen Bokat of the U.S. Chamber of Commerce. But the business community does not believe it fares that badly with the D.C. court anymore, Bokat said.

Still the push by Reagan appointees to lessen the power of the courts and enhance the power of administrative agencies can cut both ways, Bokat and others acknowledge.

For the moment, with a Republican administration and deregulation in vogue, conservatives and business groups may be happy to see the court out of the picture.

That satisfaction with a judicial hands-off approach will disappear if a different administration decides to regulate anew. Conservatives might not be able to look to Bork, Starr and Scalia for help.

Reagan's two new nominees -- likely to be former deputy attorney general Laurence Silberman and prominent local attorney Marion Edwin Harrison -- will have a profound impact on the court.

Still, there is no way to predict how the new appointees will rule.

"One thing I've learned in my 22 years here," former chief judge Carl McGowan said in a recent interview, "My colleagues continually surprise me."