Each time his turn came to question William Bradford Reynolds, Sen. Joseph R. Biden Jr. (D-Del.) did something unusual: He returned to the details of a little-known Louisiana redistricting plan that was struck down as biased against blacks.

Biden's strategy accomplished two things. It helped shift the focus of Reynolds' confirmation hearings from the murky political ground of busing and quotas to the moral high ground of voting rights.

And, when Reynolds said he had consulted the plan's black opponents and later was forced to admit he had not, it raised the first of a series of questions about Reynolds' credibility.

Biden and most of his fellow Democrats on the Senate Judiciary Committee hardly needed an excuse to vote against Reynolds' confirmation as associate attorney general. They viewed his four-year enforcement record as head of the Justice Department's Civil Rights Division as an embarrassment. But they quarreled privately over how to persuade the Republicans, whose votes they needed, that Reynolds was ignoring the law.

It has traditionally been difficult for Senate critics to block a presidential appointee on the sole grounds that they do not like his policies, which are presumed to be the policies of the president. Instead, they must produce other reasons -- lack of integrity, conflict of interest, misleading testimony -- to pronounce him unfit for high office.

That approach worked at least temporarily last week when two moderate Republicans, Arlen Specter (Pa.) and Charles McC. Mathias Jr. (Md.), joined seven of the committee's Democrats in opposing the nomination. Reynolds' supporters on the 18-member panel no longer had the votes to approve him and were forced to agree to a week's delay.

Administration supporters, led by Attorney General Edwin Meese III, are working to reverse the outcome this week. While they probably have enough votes to send the nomination to the Senate floor with no recommendation, they think that such a course would make confirmation difficult.

"I thought it was better to take a specific example of how he interpreted the law to fit his desired ends," Biden said, "rather than pick 10 or 20 or 30 cases that would allow the White House and others to argue that this is just a matter of philosophy. If you don't get deeply into the issue, then they're able to say that those Democrats are just trying to rerun the election of '84."

Biden, the panel's ranking Democrat, initially had trouble persuading his more liberal colleagues to follow his lead.

"Some of my friends on the committee and some of the civil rights groups wanted to make this an issue of busing and quotas," he said. "I thought that would be a very stupid strategy."

Despite the Democrats' criticism, Reynolds' confirmation seemed assured when he finished testifying June 5. But when the hearings were reopened last week, he faced the same complications that beset three of President Reagan's other nominees over the past year -- Meese, former Office of Personnel Management director Donald J. Devine and Leslie Lenkowsky, former deputy director of the U.S. Information Agency.

These nominees had several things in common with Reynolds. Each was an outspoken conservative who personified White House policies that the Democrats loathed. Each ran into challenges to his credibility. And each was already on the job or in another administration post, which gave detractors a sizable record to shoot at.

"It's a lot tougher when you've got a record to defend," said John H. Shenefield, who faced sharp questioning about his role as Justice Department antitrust chief when President Jimmy Carter promoted him to associate attorney general. As a first-time nominee, he said, "You say things like 'That's a very important concern' and 'I'll look at it carefully and work closely with you.' Unless you are a felon, it's much harder to block your confirmation."

Devine faced a close vote because his tenure at OPM had been marked by frequent clashes with federal workers and Congress. But he was forced to withdraw this month only after the Governmental Affairs Committee heard testimony that Devine authorized himself to act as director while awaiting reconfirmation and then asked the agency's acting director to say, incorrectly, that she had known about the action.

Meese had finished testifying at his 1984 confirmation hearings when a host of questions arose about his failure to report a $15,000 loan and his role in getting federal jobs for people who had aided him financially. Not until an independent counsel found that Meese had broken no laws did the Judiciary Committee vote to confirm him.

Lenkowsky was rejected by the Foreign Relations Committee last year after several senators charged that he lied to the panel about his knowledge of a USIA "blacklist" of liberal speakers who were barred from speaking abroad for the agency.

Reynolds' supporters have tried to blunt this line of attack. Sen. Orrin G. Hatch (R-Utah) argued that the Democrats were using "skimpy evidence" of a few ambiguities in testimony as a fig leaf for their ideological opposition to Reynolds. Hatch said Reynolds had handled about 50,000 Voting Rights Act cases and could not be expected to remember the details of each.

"I personally believe that those who oppose you have come away empty-handed," Hatch said. But he did not try to refute specific charges.

Reagan sounded a similar theme in his radio address last weekend. He said Reynolds' nomination "is being opposed by some who don't agree with us about civil rights, by some who favor the discrimination of quotas."

And Reynolds derided the civil rights establishment that bitterly opposes him as "the pro-busing/pro-quota lobby."

Faced with such criticism, Biden's staff searched for cases that exemplified what an aide called "meat-and-potatoes civil rights." With guidance from Ralph G. Neas, the politically savvy director of the Leadership Conference on Civil Rights, they pored over internal Justice Department documents during long, post-midnight sessions before the hearings began.

Biden focused on the Louisiana redistricting case in part because Jeris Leonard, who held Reynolds' job during the Nixon administration, called it "a blatant racial gerrymander." White state officials, excluding blacks from their meetings, had drawn up a plan that ruled out a majority-black congressional district in New Orleans. But Reynolds declined to challenge the plan, which was later overturned in court.

The other Democrats also stuck to voting rights. They questioned Reynolds' refusal to challenge redistricting plans in 11 Mississippi counties that were later struck down by the courts.

Reynolds, normally a combative witness, tried to appear moderate and reasonable, an approach that triggered his credibility problem. Besides saying that he had met with opponents of the Louisiana plan, Reynolds said that all the opposing lawyers in the Mississippi cases agreed that he should not enter the lawsuits. He later acknowledged he had talked to only one lawyer, who contradicted Reynolds' account in an affidavit.

Reynolds twice apologized to the panel for any misunderstanding. But by then, both Republicans and Democrats were seizing on other discrepancies in his testimony.

Biden said that Reynolds "had decided on a strategy of avoiding confrontation. He would've been better off if he had engaged us on the issue of voting rights, rather than pretend that he was truly concerned . . . . When he tried to make a case for himself, he exaggerated at a minimum, and either intentionally or unintentionally misled the committee."

The issue of Reynolds' veracity, in Biden's view, was an unexpected dividend for his critics.

"If a guy gives me a good reason to vote against him and I already don't like his philosophy, well, that's human nature," he said. "I really find repugnant Brad Reynolds' approach to civil rights. It's not legitimate to try to frame a guy because I disagree with the philosophy of the president, but I don't think that's what happened here.