Former D.C. City Council Chairman Sterling Tucker, who lost the 1978 Democratic mayoral primary by only 1,500 votes, announced last night that he would not be a candidate for mayor this year.

Tucker disclosed his decision at a mayoral candidates' forum sponsored by the Gertrude Stein Democratic Club, one of the city's major gay activist groups. He left the forum immediately after making the announcement, accompanied by his wife and two daughters, to a standing ovation.

The club later voted overwhelmingly to endorse Mayor Marion Barry for a second term after Barry and three other Democratic mayoral candidates addressed the gathering of nearly 200 persons at the First Congregational Church at 10th and G streets NW.

The other candidates included Council members Betty Ann Kane (D-At-Large) and Charlene Drew Jarvis (D-Ward 4) and Morris Harper, a Washington physician.

Tucker, who said last month that his support from the last race was still intact and that he had pledges of $250,000 for this year's race, said he would not compete in the Sept. 14 primary because the field is now too crowded for any candidate to emerge with a clear voter mandate for change.

"I have agonized over this election," Tucker said in a statement delivered at the forum last night, "trying in my heart of hearts to answer the question: What is really in the best interests of the people of this city? The crowded field of candidates troubles me greatly. And for that reason it is with great regret that I tell you: As of this moment I am no longer a candidate for the office of mayor."

As he left the church auditorium, Tucker told a reporter that he probably would support one of the remaining candidates, but declined to say which one.

He insisted that he found substantial support while he privately assessed his chances in the race. "My polls looked good, but then everybody's polls looked good," he said.

Tucker conceded that Barry must be considered a heavy favorite in a crowded field of candidates because of the power of the incumbency.

"Normally, it (the crowded field) would play into the hands of the incumbent . . . and I think normal conditions prevail," Tucker said.

Tucker has been out of local politics since he lost the 1978 mayor's race. He is now heading his own consulting firm, Sterling Tucker Associates, after leaving a federal government post last year as assistant secretary of Housing and Urban Development.

Tucker got that job at the recommendation of former HUD secretary Patricia Roberts Harris, who is herself considering running for mayor this year.

During his speech before the Stein club, Tucker said he still wants to be mayor of Washington, but felt that without a strong victory--as "the clear choice of the voter"--no future mayor could successfully govern the city. In the 1978 Democratic primary, Tucker, Barry and former mayor Walter Washington fought a close battle. Barry won with about 35 percent of the vote, while Tucker finished second with about 32 percent. Washington was last with about 30 percent.

"There's nothing I want more than to serve the people of this city as their mayor," Tucker said. "Well, yes, maybe there is: The one thing I want more is for them to have a chance to elect a mayor who is their clear choice. A choice so clear that the election will provide so much momentum to the next administration that it will be able to move the mountains that stand in our way."

The Stein club's endorsement of Barry came as no surprise. The club endorsed the mayor in 1978 and, in return, Barry has strongly supported gay-rights issues and has appointed homosexuals to several boards and commissions.

In stressing his long-standing support for the gay community, the mayor told the group last night, "You know I don't just come out of the closet at election time -- I stay out."

"I don't take support for granted," said Barry, who was accompanied by his wife, Effi. "Some candidates didn't come here tonight because they think what you're going to do is a foregone conclusion. That shows a lack of courage. Every candidate who says he's going to run for mayor ought to be here tonight."

Two announced candidates for mayor, Council members John A. Wilson (D-Ward 2) and John L. Ray (D-At-Large), declined to attend the forum. Harris, who has not yet formally announced her candidacy, also did not attend.