Polly Shackleton, the grande dame of District politics, bridled at rumors three years ago she was about to retire because of her age and then went on to win election to a third term on the D.C. City Council. Today the 75-year-old liberal Democrat from Ward 3 is preparing to retire on her own terms, and her decision already has touched off a scramble that could redefine politics in her predominantly white, middle class area of Northwest Washington.

Shackleton, a staunch ally of Mayor Marion Barry and the chief architect of much of the city's modern-day social service legislation, has not formally announced her departure, but she has told friends and associates to expect it at the end of her term next year.

"I will miss it," Shackleton said during a recent interview in her office, which is decorated with photos and mementos of a political career spanning 40 years beginning with the New Deal. "To me, it's been a very challenging and very rewarding experience over the years."

The anticipated retirement of Shackleton, the chairman of the council's Human Services Committee, already has attracted at least five serious candidates who hanker to take over her turf, including Jim Nathanson, a teacher and chairman of the Ward 3 Democrats who is expected to receive Shackleton's endorsement.

In the parlors of Ward 3, activists already are breaking out the chablis and brie for early campaign gatherings.

A bastion of white, affluent Democratic voters, as well as home to the largest concentration of Republicans in the city, Ward 3 is viewed by many as an important political battleground.

The ward was pivotal in swinging a tough Democratic mayoral primary election to Barry in 1978 and was decisive last year in the election of Republican Carol Schwartz to an at-large seat on the City Council.

"Polly has dominated the ward ever since it was a ward," explained one of her would-be successors. "With her retirement, anything can happen."

The field of candidates who have begun to campaign over a year in advance of the election includes Ruth Dixon, 63, a political scientist who ran second to Shackleton in the 1982 Democratic primary; Mary Draper Janney, 64, a civic activist, and Mark Plotkin, 38, a professional Democratic fund raiser and member of the D.C. Democratic State Committee, who also was a candidate in 1982.

Republican Gloria Corn, 37, an advocate for renters and seniors who stands more than an arm's length from her own party regulars, is also seeking the Ward 3 seat.

Candidates and political operatives are weighing the influence of Shackleton's expected endorsement of her longtime friend Nathanson, a Kennedy-style Democrat whose low-key, behind-the-scenes approach is reminiscent of the incumbent's.

Nathanson, 53, a Harvard University graduate, teaches government and law at the Duke Ellington School of the Arts.

While Nathanson covets an endorsement from Shackleton -- which likely would translate into substantial support from Ward 3 Democratic operatives -- he insists, "I am not Polly's clone."

Few say that Shackleton can control the outcome of the next ward election, and some note that previous endorsements by Shackleton have backfired. Nathanson conceded that the "coattail effect in Ward 3 is relatively insignificant."

In 1978, Shackleton endorsed former City Council chairman Sterling Tucker in the Democratic race for mayor, but Barry swept to victory in Ward 3. Four years later, Shackleton supported Barry for reelection after developing a close political relationship with him, but this time Barry lost in Ward 3 to Democratic challenger Patricia Roberts Harris while romping to victory in every other section of the city.

Shackleton's reverse coattails were at work again in 1981, when she endorsed Democrat Mary Ann Keeffe for the Ward 3 seat on the D.C. Board of Education and Republican Wanda Washburn walked away with it.

Last year, Shackleton supported Jerry Moore, a Republican at-large member of the City Council, for reelection, but Carol Schwartz, a former school board member from Ward 3, prevailed.

"The fact that I supported Mary Ann Keeffe didn't cut a whole lot of ice," said Shackleton, in recalling the school board contest. Washburn put it stronger: "I kicked Polly's machine all to hell."

"I don't think Polly has a tight ward organization," Washburn said. "She is the party guru."

Though Barry and Shackleton have remained close allies and Barry actively supported her in the 1982 election, it is unclear what role the mayor will play in the 1986 fight over the Shackleton succession.

Annette Samuels, the mayor's press secretary, said recently that Barry has made no decisions about the election.

And another Barry supporter, who was active in his Ward 3 campaign in 1982, said, "I don't know what he will plan to do next year. If you endorse a candidate , you immediately alienate all the supporters of the candidates that you don't endorse."

Barry, a former civil rights activist and member of the school board and City Council, captured the political fancy of Ward 3 Democrats during his 1978 mayoral primary battle with Tucker and incumbent mayor Walter E. Washington.

Barry received 47 percent of the ward's Democratic vote, compared to Tucker's 34 percent and Washington's 18 percent.

However, Barry's political romance with Ward 3 voters soured four years later, and he lost to Harris in practically every neighborhood in the Northwest Washington ward.

The ward, which includes such neighborhoods as Cleveland Park, Chevy Chase-D.C. and a small slice of Georgetown that includes Shackleton's home (left over after a 1982 redistricting plan eliminated most of Georgetown from her ward), is a distinct entity within the District. The ward is bounded roughly on the east by Rock Creek Park and Connecticut Avenue, on the north and west by the Maryland line and the Potomac River, and on the south by Reservoir Road.

More than 90 percent white in a city that is 70 percent black overall, Ward 3 is home to Washington's most affluent residents and has the highest concentration of elderly citizens.

Ward 3 includes more registered Republicans than any other ward, yet those 8,986 voters are only 23 percent of the ward's 38,794 registered voters. Democrats in Ward 3, as in the rest of the city, still call the tune.

For the only Republican in the race so far, Gloria Corn, the unpromising arithmetic allows her to adopt a posture of no-holds-barred antagonism toward the political establishment, especially as it is embodied in the person of Barry.

"I think the voters are ready for somebody who has the poor manners and bad taste to tell the truth," said Corn. "I think people are disgusted with the scandals that are coming out of the District Building. You know, the Teflon mayor."

If the 1982 election is any guide, Dixon has to be tabbed the favorite in the early going, a position she readily claims. "I am the frontrunner," she said, "and I have to work to stay there."

Editor and publisher of a local government newsletter called District Council Journal, Dixon earned 35 percent of the primary vote in the Democratic primary last time in a race in which Shackleton's age and health were an issue. Plotkin won 18 percent of the vote.

Dixon, a former president of the League of Women Voters who received a master's degree in political science from George Washington University, takes an intellectual approach to understanding government and boasts that she is the most knowledgable about the workings of the council.

An opponent of the rent control referendum next month who wants to boost incentives for builders in the District, Dixon drew fire from Valerie Costelloe, a Ward 3 renters advocate. On renters issues, Dixon appears to have separated herself from the pack.

Plotkin, who has fought telephone rate increases and has been an active advocate of renters, is more confrontational than all but Corn.

"The ward has always had that 'Georgetown little old lady' image," said one observer. "Mark comes on like a pugilist."

Janney, a former leader of Wider Opportunities for Women and Planned Parenthood of Metropolitan Washington, is chairman of the board of trustees at Vassar College and an admirer of the Shackleton record of traditional liberalism.

Unlike the other candidates, Janney is quick to compliment the mayor's performance, saying, "I like Barry. I think he is a compassionate person."

Nathanson, who decided in March to run and has already registered his campaign with the Office of Campaign Finance, said he believes he will have to hustle to catch up to the two.

"Even though I have filed already, I'm late," Nathanson said. "Dixon and Plotkin have basically been running for the position since the day after they lost to Polly."

Unresolved is whether it will be issues like renters rights, neighborhood development and city services that dominate the campaign or, instead, the personalities of the contenders themselves.

At this stage there is more agreement about Ward 3 voters than about the candidates or issues.

"Irascible and independent" is how Plotkin described them. "Independent thinkers," said Nathanson. "It isn't easy to influence them," said Shackleton.

They are, as one political operative put it, "somewhat fussy. The people of Ward 3 want perfect government," the operative said. "Not good government, perfect government. No mistakes allowed."

In the end, all the candidates agree, the person who wins Shackleton's seat will be the one with the best campaign organization.

"The Washington Post can't deliver the election, and the Ward 3 Democratic committee can't deliver it," said Plotkin. "Nobody but the candidate can.