Republican supporters of William Bradford Reynolds failed to muster enough votes in the Senate Judicary Committee yesterday to approve his nomination as associate attorney general, prompting the panel to delay the vote for one week.

In a major setback for the Reagan administration's leading spokesman on civil rights, Sen. Arlen Specter (R-Pa.), the key undecided member, announced that he would vote against Reynolds because the nominee had "deceived me and the committee" during sworn testimony.

That gave Reynolds' opponents at least nine votes on the 18-member committee, enough to block approval. Rather than suffer an embarrassing defeat, Reynolds' supporters agreed to a face-saving delay.

Unless someone switches sides, the best that Reynolds can now hope for is that the Republican-controlled panel will send his nomination to the Senate floor without a recommendation. That could have happened yesterday -- Specter said he would vote to allow the full Senate to decide the matter -- but the Democrats refused to go along.

Yesterday's test of strength made it clear that the allegations that Reynolds repeatedly misled the committee in sworn testimony have turned a crucial number of panel members against him. Although approval of Reynolds' nomination seemed certain 2 1/2 weeks ago, his fortunes ebbed as the debate shifted from his controversial record as head of the Justice Department's Civil Rights Division to growing questions about his credibility.

"I don't think there's any doubt he would have lost on an up-or-down vote," Sen. Howard M. Metzenbaum (D-Ohio) said of Reynolds. "It's a major setback that they couldn't get him out of committee this morning. It's a signal . . . that the administration would be well advised to withdraw the nomination."

A White House spokesman said President Reagan still believes that Reynolds "has done an outstanding job and deserves promotion."

"A very convincing case has been made that Brad Reynolds has not been enforcing the civil rights laws and has not been candid with the Senate," said Ralph G. Neas, executive director of the Leadership Conference on Civil Rights and part of a civil rights establishment that has assailed Reynolds' approach to combatting discrimination. If the nomination goes forward without a recommendation, Neas said, "there will be a very lengthy debate on the Senate floor."

No one was certain of the outcome when the committee, which has a 10-8 GOP edge, began its session. Eight of the Republicans were expected to support Reynolds, but Sen. Charles McC. Mathias Jr. (R-Md.) had indicated he would join seven Democrats in opposition.

Since a 9-to-9 tie would kill the nomination, the Republicans needed to pick up both Specter and Howell Heflin (D-Ala.), who remains undecided. Specter, who is up for reelection next year, quickly broke the suspense. Despite calls from Reagan and Attorney General Edwin Meese III, he said, he would vote against Reynolds.

Specter said Reynolds has "placed himself above the law" by disregarding court rulings on affirmative action and school desegregation. He said Reynolds had "misled" him in 1982 by implying that he opposed election laws in Burke County, Ga. -- later struck down by the Supreme Court as discriminatory -- while supporting the all-white county government in an internal memo.

Specter also said he wanted more time to look into an internal memo in which Reynolds criticized past Supreme Court rulings on school prayer as "unworthy of respect."

Committee Chairman Strom Thurmond (R-S.C.), apparently lacking the votes to do otherwise, moved to send Reynolds' name to the Senate without a recommendation. Ranking Democrat Joseph R. Biden Jr. (D-Del.) objected, saying, "The committee should not avoid its responsiblity. We should vote yes or no."

After some wrangling, the panel agreed unanimously to the delay sought by Specter and Heflin.

Mark Goodin, a spokesman for Thurmond, said the chairman wanted to give Specter more time and believes he still may decide to support Reynolds. But Specter said in an interview that he based his decision on concerns about both Reynolds' record and his misstatements, and does not expect to change his mind.

"I feel very keenly about civil rights," Specter said. "I don't think anybody's going to talk me out of my thoughts on the Burke County case, but I'm prepared to listen."