Barbara Lerner, President Reagan's nominee to be assistant education secretary for civil rights, has unexpectedly withdrawn her name from consideration, charging that the office became "impotent" after a 1984 Supreme Court decision stripped it of some powers.

That ruling, in a case involving Grove City College, has "a terrible impact on civil rights enforcement," Lerner said in a telephone interview. It allows school districts to go back to "resegregation, rank and brutal," she said, adding, "we can't let racists use tricky little gimmicks" to skirt antidiscrimination laws.

The Grove City ruling essentially said that if a section of a school discriminated -- for example, the chemistry department -- then the federal government could withhold funds only from that one section, and not from the entire school.

Lerner, a New Jersey-based education analyst and psychologist, also criticized the office for being "bogged down" with "encrusted bureaucratic paperwork."

Lerner's decision, announced in a letter to Reagan on Tuesday and made public yesterday, surprised the civil rights community and officials in the Education Department. Lerner is widely known as an opponent of quotas and affirmative action programs and for being generally in agreement with the administration's philosophy on civil rights.

"I'm overwhelmed," said Marsha Greenberger of the National Women's Law Center and a leader of a coalition seeking legislation to overturn the Grove City ruling. "I certainly think in large measure that the office's hands are tied."

"I salute her for her courage and candor and honesty," said Rep. Ted Weiss (D-N.Y.), a frequent critic of the civil rights office. "In a sense maybe it's too bad. Maybe with her strong commitment to the [civil rights] law and the need to enforce it, she could have made a difference."

In her letter withdrawing her name, Lerner said, "I was and still am honored to have been nominated by the president for that position." She said she proposed to Reagan and Education Secretary William J. Bennett an independent, bipartisan presidential commission on educational equity to review federal civil rights regulations.

The assistant secretary's job has been vacant since the end of last year, when Harry Singleton resigned after numerous clashes with members of Congress and civil rights groups that accused him of failing to follow up school discrimination complaints.

The job is now being filled, on an acting basis, by Alicia Coro, who was a Singleton deputy.

An Education Department spokesman said in a brief statement, "We will be exploring other ways that we can put her considerable talents to good use in furthering the case of equal educational opportunity."

Lerner was nominated Feb. 10. The news of her withdrawal came as many civil rights activists were preparing to criticize her nomination at the Senate Education and Labor Committee's long-delayed confirmation hearings.

Some sources in the Education Department and the civil rights community suggested that Lerner may have feared a grilling by the Senate panel, because of her past writings. She has written, for example, that activists who favor quotas to achieve "equal results" are basically "a biracial gang of . . . bureaucrats, politicians and street hustlers who make their livings off of rhetoric about racist America."