The American Bar Association, under conservative assault for rejecting Reagan administration judicial nominees, has dropped from its judicial selection panel a Washington lawyer said to be responsible for blocking several administration choices.

ABA critics of the decision, who asked not to be named, said they felt that President-elect Eugene C. Thomas, a Boise, Idaho lawyer, may have bowed to conservative pressure by not reappointing Washington attorney John D. Lane to a second three-year term on the committee. Such reappointments, while not automatic, are generally made if current members want to stay on. Lane reportedly wanted a second term.

An unfavorable report by the prestigious 14-member committee, which screens potential candidates before they are officially nominated, virtually dooms a prospective candidate's chances of nomination, and certainly of Senate confirmation, to a federal judgeship.

Conservatives have accused the committee of unfairly scuttling conservative candidates. One conservative group, the Washington Legal Foundation, has sued the committee in federal court to force it to change its ways, accusing it of meeting with "liberal and left-wing" groups such as the NAACP and the American Civil Liberties Union.

Lane, a Democrat, is being replaced as committee member for this judicial circuit by former D.C. Bar president James Bierbower, who conservative groups say will be much more responsive to the administration's goal of reshaping the federal bench.

ABA sources said Lane angered some conservatives because he was too aggressive in questioning the qualifications of some prospective candidates, including former Office of Management and Budget general counsel Michael J. Horowitz and former White House adviser Faith Ryan Whittlesey. Neither was nominated by President Reagan.

Sources said Bierbower, a low-key attorney who represented former Environmental Protection Agency toxic-waste official Rita Lavelle in her perjury trial, was not likely to be so vigorous in his investigations.

Thomas, in an interview, said there was no pressure of any sort by conservatives inside or out of the administration to remove Lane or anyone else. He also denied reports that he removed Lane because Lane had blocked the appointment of local attorney Marion Edwin Harrison, an ABA board member and friend of Thomas', to the U.S. appeals court here.

But Thomas acknowledged the Washington Legal Foundation lawsuit and said he "was concerned about that and wanted the program of judicial screening saved." He said it was "critical that we get out of this controversy."

Asked whether he thought Bierbower's appointment would accomplish that, Thomas said only that he "wanted to put the strongest people I could think of on the committee."

He said it was "absurd" to think that the ABA had a liberal bias. Lane had "done a very good job," he said, but Bierbower, a longtime friend, was "my choice." He said he wanted Lane to be chairman of another ABA committee but Lane declined.

Thomas also denied allegations by some ABA sources that Bierbower's appointment was a reward for pulling out of the race for ABA president and thus improving the electoral prospects of another conservative.

"It was my decision alone," he said. "I'd been thinking about it for months. All of this speculation is unfounded. I don't know what Lane did in blocking any nominations."

Several sources said that while Lane may have opposed some nominations, he also testified on behalf of another controversial Reagan nominee, Jay Harvey Wilkinson, who was then confirmed to a seat on the federal appeals court in Richmond.

Lane declined to comment yesterday exceptto say that "the president has the right to chose who he wants," and to confirm that he turned down Thomas' offer of a committee chairmanship.

Robert B. Fiske Jr., chairman of the selection committee, said Lane "was a valuable member of the committee who did excellent work" handling a number of difficult cases.

D.C. Bar President Paul Friedman said he had "taken it upon myself" to write Thomas recently and ask that Lane be reappointed because he had done a "terrific and thorough job."

Various administrations informally have used the ABA to assess candidates before they are nominated in order to spot potential confirmation problems and save both the nominee and the administration political or personal embarrassment.

Conservatives have been particularly bitter, however, about the ABA's resistance to a number of nominees, especially University of Texas law professor Lino Graglia, former Legal Services Corp. chairman William F. Harvey. Harvey has accused the ABA of opposing his nomination because he refused to fund the organization while head of Legal Services.