The Washington Post's endorsement yesterday of D.C. City Council member Marion Barry for mayor sent supporters of all three major candidates in the Sept. 12 Democratic primary scurrying to refine their political strategies.

Barry's organization, which for months has been trying to climb out of the basement of the three-way race, greeted the endorsement as an indication that Barry, long plagued by in-age problems, had been given increased respectability.

Barry campaign manager Ivanhoe Donaldson said the endorsement would help Barry win some of the undecided voters who are considered crucial in the close race of the nomination.

Supporters of City Council Chairman Sterling Tucker responded to the endorsement by declaring publicly for the first time that Barry was a "spoiler" in the race - a candidate who cannot win because he is hopelessly behind.

"At his point in the race, two weeks before the election and with Walter Washington and Sterling Tucker in a dead heat, a vote for Marion is a vote for Walter Washington," said Del. Walter E. Fauntroy, who has endorsed Tucker.

The endorsement of Barry was seen as a plus by some in Washington's organization, where the mayor's reelection strategy has been premised on Tucker and Barry splitting the anti-Washington vote.

Washington, whose administration was sharply criticized by the newspaper the day after he announced his decision to seek reelection, had not expected to win its endorsement. But the newspaper's support of Barry helps Washington's candidacy, Washington strategists said, because it improved Barry's chance somewhat and also was a criticism of Tucker.

The endorsement by The Post marked the first time since elected local politics returned to the city in 1974 that the newspaper had endorsed a candidate for mayor.

Yesterday's endorsement capped a three-part series of editorials that first explained why neither Washington, Dorothy Maultsby or John Ray would be endorsed. The final editorial explained the choice between Tucker and Barry.

"What Mr. Barry seems to value, and to be offering . . ." the editorial said, "is precisely what we think the people of this city need . . . energy, nerve, initiative, imagination, toughness of mind, an active concern for people in distress, command presence, if you will - (qualities of leadership) that have been conspicuously absent in the present administration, and that also seem to be missing from Mr. Tucker.

"To put it bluntly, on this matter we do not see that much difference between Sterling Tucker and Walter Washington."

Philip L. Geyelin, editorial page editor of The Post, said each of the three major candidates met separately with the 10 members of the editorial page staff and Post Publisher Katharine Graham on July 26, 27 and 28. The editorial page staff then decided on the endorsement, he said.

The endorsement of Barry was published on the same day that Tucker came to a previously scheduled luncheon interview with editors and reporters from the metropolitan staff of the newspaper. Tucker had orginally been scheduled to meet with the group last week, but the luncheon was postponed when he went to California to lobby on behalf of a constitutional amendment to give the District full voting representation in Congress.

Herbert H. Denton, the Post's District of Columbia editor, stressed yesterday that there is a strict separation between the newspaper's editorial page staff, and the news staff, which is covering the election.

"We had nothing to do with the endorsement," Denton said, "and it will in no way affect our coverage of the campaign. It is an unfortunate coincidence, caused by Tucker's absence from the city late last week, that his luncheon interview occured on the same day that the editorial page endorsement was published."

When asked about the endorsement at the luncheon, Tucker responded, "I would have accepted the endorsement had it been offered, and would have welcomed it as I have all the other endorsements I received. And I think it will be regarded as one more element among many in the overall campaign."

Washington's campaign staff issued a brief statement on his behalf. "The post is entitled to its views," the statement reads. "I'm working very hard to get the endorsement of the voters on Sept. 12."

Throughout the campaign, Barry, a former codirector of Pride Inc., has suffered from his past image as a loud and dissident black activist who at times stood as the symbol of opposition to the city's established order.

Campaign manager Donaldson said yesterday that the Post endorsement would help to offset that image and could shore up support for Barry among undecided voters.

"It shows," Donaldson said of the endorsement, "that another institution in the community feels that not only is Marion well qualified, but he represents the kind of change the city needs."

"The more people get a sense of credibility about a candidate," Donaldson said, "the more they begin to think about (voting for) the candidate."

Donaldson said the Barry organization plans to distribute fliers publicizing the endorsement and is considering incorporating it into Barry's television and radio ads.