D.C. Mayor Marion Barry announced last night he will not seek a fourth term, signaling the end to his dominance over District government and politics as his federal trial on perjury and cocaine charges enters a critical phase.

In a pre-recorded 15-minute address broadcast by Howard University's television and radio stations, Barry said he was abandoning plans to run this year for another four-year term so that his "extended family" of District residents could begin healing "politically."

"Accept my word that stepping aside now is good for our city," said Barry, 54, who was first elected mayor in 1978. Later, as he finished his speech, he said: "Tonight it's time to cast away."

After the eighth day of jury selection in his trial concluded yesterday, Barry said he would not resign as mayor before his current term ends on Jan. 2, 1991, and he stressed that his political decision was "not related to my legal situation."

However, a number of close Barry associates said his announcement was part of a defense strategy to paint the mayor as a defeated politician and an unworthy target for the full force of the law.

According to sources, Barry's defense lawyers have decided on this course because U.S. Attorney Jay B. Stephens has been unwilling to conduct plea negotiations.

The speech was timed to take place before jury selection concludes on Monday, the sources said. Once impaneled, jurors will be sequestered, with no possible access to media reports about the mayor's case or his political plans.

Barry, dressed in a dark suit, striped shirt and colorful tie, appeared composed as he sat alone in a studio and delivered what in many ways was his political valediction. He quoted Scripture and recited a list of Biblical figures, and took pains to recall the good works of his administration, such as programs for the elderly and the disenfranchised, the "kept out, locked out and shut out" residents of Washington.

He also referred repeatedly to his own struggle with what he has described as an addiction to alcohol and prescription drugs, saying that while many of his supporters believed that he could be reelected, the office itself was not as precious as a triumph over addiction.

"My supporters and political polls have indicated that if I were to run, I could win," Barry said. "But what good does it do to win the battle, if in the process I lose my soul?"

Barry repeated his steadfast opposition to resigning, saying it would be too "disruptive" to the District and its politics. By taking himself out of the Sept. 11 Democratic primary, Barry may free up some of the financial contributors and campaign volunteers who have been sitting out the mayoral primary, waiting for him to decide on his candidacy.

Barry repeatedly suggested that he intends to try to influence the outcome of the mayor's race, and urged his supporters to await word from him before deciding whom to support. One group of Barry supporters has scheduled a two-hour rally today in his honor outside U.S. District Court, where the mayor's case is being tried.

In an interview with WHMM-TV after his speech yesterday, Barry also needled D.C. Del. Walter E. Fauntroy (D), a mayoral candidate, for calling on the mayor and Stephens last month to negotiate a plea agreement.

Barry contrasted Fauntroy's actions with those of Jesse L. Jackson, whom he praised for consulting frequently with him in recent days and acting as an "interpreter" for the mayor. Jackson said last week that a decision by Barry not to run might be an important step toward reaching a plea agreement with Stephens.

Barry said Jackson "has been helpful" but Fauntroy, who spoke out without first consulting with the mayor, "has been hurtful."

Fauntroy said last night that he was surprised by Barry's "harsh feelings" toward him for making essentially the same appeal as Jackson. Fauntroy said he hoped to meet with the mayor to discuss their differences.

"I intend to reach out to Marion Barry the man to be of whatever assistance I can," Fauntroy said.

Other mayoral candidates said they welcomed Barry's decision, in part because it could redirect the city's focus from the mayor's troubles to the campaign itself and issues confronting the District.

"He's done the right thing," said D.C. Council Chairman David A. Clarke (D).

Sharon Pratt Dixon, the only Democratic mayoral candidate to call for Barry's resignation, said the mayor's announcement "comes too late." If Barry had announced sooner that he would not run, Dixon said, "Maybe we would have been spared the trial and this whole ordeal."

Council member Charlene Drew Jarvis (D-Ward 4) said Barry's decision "will allow voters to focus on the issue of the candidates who want to serve them."

Democrat John Ray, an at-large council member, said, "This is Marion Barry at his best . . . . He is enabling the political process to move forward, unencumbered by the turmoil which surrounds his present legal difficulties."

Republican mayoral candidate Maurice T. Turner Jr., a former D.C. police chief said, Barry's decision would be "healthy for the city . . . We need to go and get new leadership in place."

National black political leaders attending the Bethune-Du Bois Fund dinner at the Omni Shoreham Hotel last night expressed both support for and sadness at the mayor's decision.

"The mayor's decision was correct and courageous," said Jackson, who moved to the District last summer and briefly considered running for mayor this year.

"He has wrestled with the situation," Jackson said of Barry. "He has counted the costs, he came to a conclusion to cut the losses. He chose to make a statement to spare people the pain and to put his focus on the trial."

New York City Mayor David N. Dinkins called the decision "a very personal thing," and said it would appear to some that Barry made a "judgment in the best interest of his family."

Barry acknowledged that there were some who would cheer his announcement, but said in effect that D.C. voters would appreciate him more after they take a closer look at the field of mayoral candidates.

"It's easy to say, 'goodbye Marion Barry,' but you've got to look at who's coming on the scene," he said. "I predict some of the same people who are now cheering may not cheer after a while."

Since his Jan. 18 arrest in an FBI sting operation at the Vista Hotel, Barry has contended several times that federal authorities were trying to drive him from office. The mayor has described the government's prosecution as a "political lynching," saying in one interview that authorities tried to "kill" him by allowing him to smoke crack cocaine at the Vista.

In his speech last night, Barry did not directly criticize the role of the U.S. government, reserving his harshest remarks for the news media, which he said were "obsessed" with his private life.

Barry also said he hoped his announcement would lift the "shadow" of uncertainty that his trial had cast over himself and the announced candidates.

"The shadow of these circumstances should be removed so the process of election of government can proceed with the highest priority it deserves," Barry said.

Barry said he chose the Howard University stations as his forum out of symbolic and practical concerns, to pay tribute to that historically black institution and to avoid "rude and disrespectful" members of the news media.

In a gesture to Washington's several black-owned newspapers, Barry taped his speech about noon and then gave a copy of the text to several executives from those weeklies so they would have the news for their Thursday editions.

Although he suffered extraordinary political damage with his arrest, which occurred only three days before he was to announce his candidacy for a fourth term, Barry and his staunchest supporters seemed to believe for some time afterward that he might be able to resurrect his mayoral bid.

There was some evidence to support that view, including several polls that showed a solid core of support -- about 25 percent of the electorate -- for Barry. In addition, the mayor himself staged a dramatic reentry to the District in March after seven weeks of addiction treatment in Florida and South Carolina, and then launched a spirited public relations campaign that his Democratic rivals had neither the resources nor the time to wage.

As his trial drew closer, though, Barry began talking, sometimes openly, about the possibility of life after politics. For a man who relishes the perquisites of political power as Barry does, the statements represented a remarkable change of attitude.

Last night, there was a glimmer of the tough, big-city mayor in Barry's address and in a later interview with Kojo Nnamdi, a talk show host on Howard's WHMM-TV (Channel 32) as he told his supporters to await instructions from him.

Although Barry seemed resolute in his decision not to run for mayor, he mentioned none of the lesser offices that are up for grabs in the fall elections, some of which his strategists have said he may consider seeking. Barry also has noted that any formal or informal role for him in politics will hinge largely on the outcome of his trial.

Barry's career in politics, first as a civil rights organizer and later in elective office, has spanned nearly three decades.

He served as the first national chairman of the Student Non-Violent Coordinating Committee, the civil rights group that played a pivotal role in the desegregation of the South. In 1967, he helped start Youth Pride Inc., which trained a number of Washington's poor and unemployed residents.

In 1971, Barry ran for a seat on the D.C. Board of Education because he believed, he told an interviewer last month, "I could be more effective on the inside" of the political process than outside as a community organizer.

Barry showed a considerable talent for politics, and in 1978 he upset incumbent Walter E. Washington and then-council Chairman Sterling Tucker in the Democratic mayoral primary. Then, as now, the District's electorate was overwhelmingly Democratic, and Barry went on to an easy victory in the general election that fall.

Barry's early years as mayor were widely regarded as productive ones. He brought order to the city government's fiscal house, greatly expanded city contracting opportunities for minorities, and presided over the beginnings of a construction boom that would remake the face of downtown.

Barry's early success in politics was in part a tribute to his skill at coalition building -- his ability to bring together such diverse constituencies as blacks, labor activists, gay people and whites.

Later on, as reports of drug use by Barry and corruption within city government began surfacing, some key voter groups, notably whites in the precincts of Northwest Washington, deserted the mayor in large numbers. Nevertheless, he won reelection handily in 1982 and again in 1986.

In a series of interviews last month before the start of his trial, Barry defended his tenure as mayor, portraying himself as a generous chief executive who made sure that all D.C. residents shared in the city's wealth and resources.

For instance, he told radio station WAMU-FM: "I think what's important when you look at Marion Barry's 12 years of service to this city as mayor -- four years on city council, three years on the school board and almost 30 years collectively in the civil rights-human rights-political arena . . . we've made so many contributions, we've done so much for so many, whether it's our senior citizens or our young people, or a summer job for everyone, whether it's downtown development, whether it's streets and potholes."