Conservative activists were certain that William F. Harvey, former chairman of Legal Services Corp., would have no problem winning approval to be a federal judge. But Harvey dropped out of contention after his proposed nomination ran into trouble with the American Bar Association.

When the ABA's screening committee rejected Lino A. Graglia, a University of Texas law professor and outspoken foe of school busing, the Reagan administration asked the panel to reconsider. Not a word has since been heard about the nomination.

Since 1952, when the bar association began evaluating potential judges for the White House, the process has been cloaked in secrecy. But angry conservatives are trying to throw a spotlight on the ABA's backstage role.

In a recent lawsuit, the conservative Washington Legal Foundation charged that the ABA screening panel acts as a federal advisory committee and is breaking the law by meeting behind closed doors.

"They operate in secret, Star Chamber-like proceedings in collusion with liberal, left-wing groups," said Paul D. Kamenar, the legal foundation's executive director.

But one liberal group, People for the American Way, called the suit "a blatant attempt to intimidate the American Bar Association . . . . The far right has a beef with the ABA because it has not been rubber-stamping unqualified, extremist candidates."

The lawsuit comes as liberals are loudly criticizing President Reagan for filling the federal bench with conservatives. Although Reagan has named more than 250 of the nation's 743 federal judges, conservatives are stewing about the candidates that never made it past the ABA.

The president, who seeks the ABA's stamp of approval before making a nomination, is not bound by its recommendations, but in practice a rejection by the ABA almost always kills a nomination. Reagan has nominated only one candidate rejected by the ABA, former Commerce Department official Sherman Unger, who died before the Senate Judiciary Committee acted on his nomination.

The ABA committee investigates the candidates' backgrounds, reviews their writings, interviews colleagues and rates them on a four-part scale from "exceptionally well-qualified" to "not qualified."

Its review is officially limited to competence, integrity and judicial temperament, but those categories are broad enough to allow considerable lobbying by political activists.

The legal foundation's suit has forced a change at the ABA, which had been seeking comments on prospective nominees from such liberal groups as People for the American Way and the Alliance for Justice.

After the Washington Legal Foundation demanded the candidates' names, the ABA committee said it would no longer share the names with any outside group.

"We were cut off . . . because of pressure by the administration," said Alliance for Justice director Nan Aron. "The administration would like to have carte blanche in this area and not have to submit names to the ABA committee."

ABA officials declined comment on the suit. But the chairman of the ABA committee, New York attorney Robert B. Fiske Jr., told Aron in a Dec. 3 letter that the administration does not want the names of potential judges made public.

"The president should be able to obtain a confidential assessment of prospective nominees," Fiske wrote. " . . . It is also important that prospective nominees not ultimately nominated should be spared as much embarrassment as possible resulting from the committee's investigation."

Grover Rees III, special assistant for judicial selection to Attorney General Edwin Meese III, said the Justice Department was not consulted about the lawsuit.

But he said any conservative would be "pretty upset" about the ABA consulting with liberal groups.

"It was very unfortunate . . . that they were sending the names to politically or ideologically motivated groups, whose goal is to slow down and stop our nominees by whatever means they can," Rees said. "These groups want to see that Ronald Reagan gets to appoint as few judges as possible that think like Ronald Reagan."

Rees said that the bar association does not have veto power over judges and must be careful not to stray into questions of ideology. "They are asked for their advice on a narrow range of issues related to professional competence," Rees said, adding that too many candidates are rated poorly "because they did not spend their life as trial attorneys."

Rees added that a 1974 opinion by Antonin Scalia -- then the Justice Department's chief legal adviser and now a conservative favorite on the U.S. Court of Appeals here -- found that the ABA panel was not covered by the law requiring federal advisory committees to meet in public.

Still, a number of liberals agree with the lawsuit's aim of bringing the ABA deliberations into the sunlight. Sen. Paul Simon (D-Ill.), a member of the Senate Judiciary Committee, has said the Senate is entitled to know why the ABA rates some nominees as minimally qualified.

"I'd love to know what their ratings are based on," Aron said. "I would love for the process to be open." But even without formal notification, she said, word quickly leaks out about which candidates are undergoing ABA scrutiny.

Susan Liss of People for the American Way disputed the legal foundation's charge that the ABA panel is too liberal, noting that it has approved most of Reagan's conservative nominees. "If the ABA committee were a bunch of left-wing radicals, they would never let the vast majority of these names through," she said.