The nation's civil rights lobby, viewed as a diminished force in Washington a few years ago, has enjoyed extraordinary success over the last year in blocking the Reagan administration's initiatives on civil rights.

Whether it is fighting presidential nominees or policy changes, this coalition of civil rights activists and liberal groups, now in a defensive role, has a string of victories:

*The Senate Judiciary Committee rejection last June 27 of Reagan administration civil rights chief William Bradford Reynolds' nomination to be associate attorney general.

*The leak last Aug. 14 of a Justice Department plan to abolish minority hiring goals for federal contractors, which sparked an outcry that has kept the proposal on ice for nine months.

*The Senate Judiciary Committee refusal May 8 to approve judicial nominee Daniel A. Manion, sending his name to the Senate floor with no recommendation.

*The rejection May 20 by another Senate committee of Jeffrey I. Zuckerman's nomination to be general counsel of the Equal Employment Opportunity Commission.

*The Judiciary Committee rejection last Thursday of Jefferson Beauregard Sessions III, for the first time killing one of President Reagan's judicial nominations.

To be sure, the civil rights movement does not have the clout of years past and has been unable to implement much of its legislative agenda. It has failed for two years, for example, to secure legislation overturning the Supreme Court's 1984 Grove City College decision, which restricted federal antibias enforcement at institutions of higher education that receive federal funding.

But when it comes to administration initiatives, such liberal and civil rights groups as the Leadership Conference on Civil Rights, People for the American Way, Alliance for Justice and NAACP Legal Defense Fund have proved tenacious adversaries.

"It is an effective, hard-hitting coalition," said Patrick B. McGuigan of the conservative Free Congress Foundation, which fought for Reynolds, Manion and Sessions. But he said he is "disgusted at the radical civil rights lobby and what they're able to do to decent human beings."

McGuigan's comments were mild compared to those of Attorney General Edwin Meese III. Meese issued an angry statement Thursday after Sessions, the U.S. attorney in Mobile, Ala., was voted down 10 to 8 for a federal judgeship in his state on grounds that he has been insensitive to civil rights.

Calling the vote "an appalling surrender to the politics of ideology," Meese blamed it on "several liberal organizations" that engaged in "vicious and highly personal attacks" and tactics he termed "reprehensible" and "repulsive."

"The attorney general gives us this power by consistently putting forth unqualified people for high positions," said Anthony T. Podesta, president of People for the American Way, which waged a mass-media campaign against Sessions and Manion. "It is his extremism that enables groups like People for the American Way to make a case against what he wants to do. In his search for an ideologically pure judiciary, he provides us with ready targets."

"The radical right has not been able to pass one single item on its agenda in the last five years," said Ralph G. Neas, executive director of the 36-year-old Leadership Conference. "The last year in particular has been a rejection of extremism. The right-wing ideologues who control the Department of Justice have been isolated on issue after issue."

Neas said the civil rights movement, with its history of support from churches and Jewish organizations, has always relied on "moral suasion" instead of arm-twisting.

But it also has lobbyists such as Neas who "are in tune with what flies up on the Hill," a Democratic Senate aide said. "They understand the Hill in a way that the Justice Department doesn't."

In policy terms, the effect of these victories is unclear. Several senators said Reynolds' defeat last year would signal the administration that its handling of civil rights was unacceptable, but there has been no discernible change in policy. While balking at the Sessions and Manion nominations, the Senate has approved more than 250 other Reagan judges who share the president's conservative philosophy.

In political terms, however, there is a definite fallout. A Justice Department official said the civil rights lobby keeps winning by picking up the votes of two Judiciary Committee Republicans, retiring Sen. Charles McC. Mathias Jr. (Md.) and Sen. Arlen Specter (Pa.), who is in a tough reelection race.

At the same time, he said, the Democrats "have been especially good at enforcing party discipline" this year, even on non-civil rights nominees such as Donald J. Devine, who was forced to withdraw his renomination as director of the Office of Personnel Management.

Noting that the White House made little effort on Sessions' behalf, this official said: "Don't they know what's at stake? This is the first Reagan judge rejected by the Senate Judiciary Committee. It's having an effect on the position of the president."

"The Justice Department is working hard, and people like me are working hard," McGuigan said. "I want to know where the White House is."

The battle over the judiciary has come to the forefront for several reasons. One is that Reagan, whose nominees will account for half the nation's federal judges, is transforming the bench in a way that will far outlast his presidency. Another is that critics contend that Meese has put forth more extreme and marginally qualified nominees in Reagan's second term.

Liberal activists decided last year to marshal their resources against Reagan's judicial choices. But they deny Meese's charge that their opposition is purely ideological. "We don't oppose people whose views we disagree with, if we feel they would be fair-minded and independent on the bench," said Nan Aron of Alliance for Justice.

Aron said the defeat of Sessions, who was accused of making racially insensitive remarks, is significant because "it draws a line" on future nominations. She said the administration now will be less likely to resurrect such potential nominees to the bench as William F. Harvey, former Legal Services Corp. chairman, and Lino Graglia, a University of Texas professor.

Preemptive tactics succeeded last summer when the administration dropped plans to nominate Herbert E. Ellingwood, an evangelical Christian who then headed the Merit Systems Protection Board, to be the Justice Department's chief adviser on judicial nominees. Ellingwood was attacked in an advertising blitz by People for the American Way, the well-financed lobby founded in 1980 by television producer Norman Lear.

In each civil rights dispute, the outcome often rests on which side can claim the political mainstream. A Meese-Reynolds executive order on affirmative action, for example, has been bottled up since opponents obtained strong backing in Congress and in Reagan's Cabinet.

To block nominations, the most liberal Judiciary Committee Democrats -- Edward M. Kennedy (Mass.), Joseph R. Biden Jr. (Del.) and Howard M. Metzenbaum (Ohio) -- must hold their more conservative colleagues as well as pick up GOP votes. Sessions' defeat was not sealed until the panel's most conservative Democrat, Sen. Howell Heflin of Alabama, voted against his home-state nominee.

Despite the yearlong string of victories, most liberal lobbyists remain on the defensive. "For the most part," Podesta said, "our role in Washington has to do with stopping bad things from happening."