An FBI background investigation is focusing on several potential controversies over the expected nomination of Herbert E. Ellingwood to head the Justice Department's Office of Legal Policy, which screens candidates for federal judgeships.

Opponents have mounted a preemptive campaign to defeat Ellingwood, a longtime associate of Attorney General Edwin Meese III and now chairman of the Merit Systems Protection Board.

One reason for their opposition involves questions about Ellingwood's connection with a "Christian talent bank" to recruit Christians for federal jobs.

Another concern is based on allegations that he helped arrange a federal job for the wife of a mutual friend who had given Meese's wife a $15,000 interest-free loan.

Ellingwood declined to be interviewed for this article.

If nominated, Ellingwood would help President Reagan select 114 federal judges -- more than one-seventh of the judiciary -- at a time when some have accused the administration of screening candidates according to their views on such issues as abortion.

An FBI background check is usually the last step before a presidential nomination. But some sources say the administration is having second thoughts about facing another major confirmation fight in the Senate Judiciary Committee, which last month rejected William Bradford Reynolds' nomination to be associate attorney general.

Among those interviewed by the FBI are aides to Rep. Patricia Schroeder (D-Colo.), chairman of a House Post Office and Civil Service subcommittee that has investigated Ellingwood.

FBI agents also have interviewed officials at People for the American Way, a nonprofit liberal group.

One official, Melanne Verveer, said she questioned whether Ellingwood "would impose a religious test" on judicial candidates.

Ellingwood, who was Reagan's legal affairs secretary in California and a deputy White House counsel before taking over the merit board in late 1981, makes no secret of his religious activities. He is a regent of the Christian Broadcasting Network University, holds prayer meetings in his office and speaks to Christian groups around the country.

The General Accounting Office found last year that he had earned $16,000 in honoraria over a 20-month period, mostly for speaking to Christian groups. Seven of Ellingwood's speaking engagements coincided with his official travel, but the GAO said he made the speeches on personal time.

Ellingwood once said that Reagan, while governor of California, was "instantly healed of ulcers" after being told that a Sacramento prayer group was praying for him. Ellingwood called it "a miracle," according to the Los Angeles Times.

In 1982, according to The Sacramento Bee, he told a church group:

"Where have you -- the church -- been when 13 million babies have been killed by abortion? Where have we been when we've allowed . . . hard-core pornography to come even onto our own television sets? I think every Christian ought to be involved in politics."

The Federal Times reported in January that Ellingwood was providing help to a Christian "talent bank" operated by American Coalition for Traditional Values (ACTV), a network of about 100,000 fundamentalist churches.

Curtis Maynard, ACTV's executive director, told the newspaper that Ellingwood notifies the group of "openings in the civil service" and "channels" applicants to "the right place." ACTV President Tim LaHaye was quoted as saying the program aimed to "flood the bureaucracy with Christians."

In response to an inquiry from Schroeder, Ellingwood said he has never advised ACTV on civil service openings. He said that after learning that the group had "an advisory service" on civil service vacancies, "I did suggest to Dr. LaHaye that he include a similar service for those who might be interested in a presidential appointment."

Ellingwood said he thinks competent people should be involved in government "regardless of their religious affiliation." LaHaye, in a separate response, said the program involves distributing brochures and public information about federal job vacancies.

Schroeder said Ellingwood has displayed "a total disrespect for the whole idea of the merit system. He seems to think Christians have a corner on decency. You really have the feeling he thinks he's answering to a higher law."

Schroeder also criticized Ellingwood for allegedly arranging a merit board job for Gretchen W. Thomas, an issue that surfaced during an independent counsel's probe of Meese last year.

Thomas, who had been a White House intern under Ellingwood, sought his aid when her husband, Edwin, was made San Francisco regional administrator for the General Services Administration in 1982. The appointment followed a $15,000 loan from Edwin Thomas to Meese's wife in 1980.

A report by Schroeder's staff said that Ellingwood lobbied several merit board officials to hire Gretchen Thomas, who had just graduated law school. She was later given a $23,566 attorney's job that was created for her in the merit board's San Francisco office.

A. Luis Lucero, the agency's San Francisco regional director, said he told Ellingwood that Thomas was not qualified for any job in his office, according to the report. But Lucero said Ellingwood told him that this did not matter and that he should "back off," the report said.

Ellingwood said at the time that he had not lobbied agency officials on Thomas' behalf or violated merit hiring principles.

Four of the approximately 18 employes in the agency's San Francisco office, including three blacks, were laid off last month in a reduction in force, but Thomas was not among them.