It was billed as the turning point of the increasingly intense campaign for the Democratic nomination for mayor.

Polly Shackleton, the grand dame of the District of Columbia's politically potent Ward 3, would endorse Sterling Tucker for mayor, an event Tucker's press secretary predicted would have a "shattering impact" on the campaign and swing the election to Tucker.

But it did not happen.

Instead, a somewhat nervous-looking Shackleton strode into the City Council chambers at the District Building, sat down at a table in front of radio, television and newspaper reporters and more than 100 stalwart Tucker supporters and said nothing.

Shackleton's anticipated endorsement of Tucker was to have been the final round of nearly a week of tug-of-war between supporters of Tucker and Marion Barry, both of whom are trying to unseat Mayor Walter E. Washington in Tuesday's primary.

Barry has strong support in Ward 3, the mostly white and affluent section of the city west of Rock Creek Park, but is significantly behind Tucker citywide and Washington in many polls.

Since earlier this week, Tucker supporters have been trying to capitalize on what they believe is strong anti-Washington sentiment in the ward by urging those who do not want to see Washington reelected to vote for Tucker.

Shackleton, who in 10 years as a council member and community activist has assembled a strong following in the ward and established such a reputation that her word can swing some undecided voters, was the focal point of that struggle.

The only statement made at yesterday's aborted press conference was read by the Rev. David Eaton, senior minister of All Souls Unitarian Church, who along with golfer Lee Elder and Elder's wife, the Rev. William Wendt and Shackleton, were to endorse Tucker.

"We had originally called a press conference today at 3 p.m. to discuss the mayoral elections," Eaton said. "The conference has been rescheduled for 11 a.m. tomorrow morning.

"We now have reason to believe a more complete statement can be made tomorrow. We have no comment beyond this statement."

Shackleton and Tucker declined further comment afterwards.

According to several Barry supporters, the announcement was postponed because Eaton, who is also Barry's minister, had been persuaded temporarily to postpone his decision.

The key persons influencing Eaton, the sources said, were Julius Mack, a member of Eaton's congregation who is also active in city political circles, and Herbert O. Reid, a Howard University law professor who is one of Barry's closest advisers.

According to the sources, Reid and Mack told Eaton that his endorsement of Tucker would make him party to the efforts of D.C. Del. Walter E. Fauntroy, a strong Tucker supporter, and others to become "kingmakers" in the local Democratic party.

At the same time that Reid and Mack were lobbying Eaton, the source said, some members of Barry's campaign staff who are very close to Shackleton - including Mary Lampson, Shackleton's former campaign manager - called a top Shackleton aide to remind Shackleton of a recent statement in which Shackleton said she would "never" endorse Tucker.

The Barry sources said the aim of the lobbying effort was to embarrass Tucker by having the highly publicized announcement delayed.

After the announcement, one ranking Tucker campaign aide lamented privately that the effect of the delay would undoubtedly be to "not help" the Tucker effort because it would "create uncertainty."

Shackleton has actively campaigned against Washington's reelection. But her own former council staff and campaign workers are sharply divided between Tucker and Barry, so she has not sided with either.

Increasingly in the past several days, however, as polls began to indicate that Barry was significantly behind Tucker and Washington, Tucker supporters began to lean heavily on Shackleton to come out in support of Tucker.

Barry was too far behind to win and votes cast for him would be wasted, they argued. That would result in a split in the anti-Washington sentiments that Tucker people believe are overwhelming in Ward 3, and lead to the reelection of Washington in the close, three-way contest, Tucker supporters said.

Before the nonannouncement, Tucker campaign workers were trumpeting Shackleton's expected support for Tucker as a major breakthrough that would carry Tucker to victory.

"It will have a shattering impact on the campaign," campaign spokesman Sherwood Ross had proclaimed. "The swing is on to Sterling."

"When you see the number (in the Post's poll showing Tucker with a slight lead) and add Polly, bang, you got 'em." said Tucker's Ward 3 coordinator, H. Crane Miller, a lawyer.

"I would hope the impact of Polly would be enough to carry the election," Miller said. "Repeadtedly in our canvassing (in Ward 3) we have had people ask us, 'Where's Polly on this?'"

"We've had to say she hadn't made a choice. It took enormous political courage" for her to endorse Tucker, Miller said before the press conference.

So certain were the Tucker supporters of the impact of a Shackleton endorsement that they had already made plans to check the first edition of today's newspapers to see if published accounts of her endorsement were "acceptable" to reprint and send to undecided voters, Miller said.

Even so, Miller said the Tucker campaign plans to send flyers to 5,000 undecided voters in Ward 3 with a reprint of The Post's poll results with the following message in capital letters:

"BY SPLITTING VOTES BETWEEN TUCKER AND BARRY, DEMOCRATS WHO DON'T WANT ANY MORE OF WALTER WASHINGTON'S ADMINISTRATION MAY END UP REELECTING WALTER WASHINGTON."

The letter goes on to say that the results of the poll say "clearly that it is Sterling Tucker who has the one good chance to defeat Walter Washington.

"We're asking you to vote for Tucker . . . even if you've been leaning toward Barry. Both men have done good things for our city. But it won't help either of them, or you, or us, or our city in next Wednesday's headline reads: WALTER WASHINGTON FOR FOUR MORE YEARS.

"In Ward 3 you can't get people out to rallies and they hate sound trucks" blaring candidates' promises up and down their quiet residential streets, Miller said. Moreover, he said that trying to reach voters at grocery stores is often times fruitless because people from outside immediate neighborhood and non-D.C. residents often shop there.

"We've made 11,000 phone attempts (to reach Ward 3 voters) and reached 5,000," Miller said. "Six thousand of them were on vacation."