The day had barely begun at the stuffy, unair-conditioned one-time drug store that serves as D.C. City's Council Chairman Sterling Tucker's mayoral campaign headquarters when his press secretary, Sherwood Ross, walked in with an early copy of the afternoon newspaper.

A few minutes later, Ross walked over to another campaign aide, general counsel Harley J. Daniels, and, with a mischievous look on his face, silently pointed to a pair of large headlines over stories critical of the D.C. government. Daniels nodded his head and Ross announced: "I'll get right on it."

Within a couple of days the Tucker campaign had cranked out another broadside at the administration of Mayor Walter E. Washington, with Tucker saying that the failure to appoint a state health planning agency director for the city was another example of the "drift, delay and indecision that are the hallmarks of its inefficient administration."

It is the Tucker staff's attention to any detail that might reap a political advantage for their candidate, as well as Tucker's repeated attacks on the mayor for his alleged mismanagement of the city government, that have become the linchpins of Tucker's bid for a lease on the mayor's office for the next four years.

It is a bid that Tucker and his campaign aides feel he is winning at the moment, a point coceded by Ivanhoe Donaldson, campaign manager for the third principal contender in the Sept. 12 Democratic mayoral primary, city councilman Marion Barry.

But it is a race that all three candidates think they can win and one that by most accounts will hinge on which way the still hefty percentage of undecided voters turn on election day.

A Washington Post poll in early June showed Tucker the choice of 24 percent of the 1,020 registered democrats polled, with 20 percent for Washington and 18 percent for Barry. But the largest group of voters, 35 percent, was undecided.

More recent polls by various city candidates have placed the size of the undecided block in the 10 to 15 percent range, a smaller but still decisive group of voters.

Tucker, convinced he is ahead in the race, says that all he has to do to win is collect his share of the undecided group while holding on to his committed supporters. With that in mind, the Tucker campaign for weeks has tediously pored over lists of registered Democratic voters in the city and canvassed them to pinpoint likely Tucker voters.

Canvassing is common in most campaigns, but the Tucker camp seems to have carried it a bit further than most. His workers plan to have a computerized list of would-be Tucker voters by election day and then call each of them to make sure they get to the polls.

"It's a systematic campaign," says Daniels. "It reflects the way Sterling does things."

Tucker often participates in the canvassing effort, hiking from door-to-door to meet voters his staff has determined fall into the undecided category. At other times, Tucker has walked from room to room at the downtown offices of Teamsters Local 639 as volunteers have made calls on his behalf. When one of the aides reaches a voter who is undecided or who represents a "soft" Tucker vote, the candidate gets on the phone and makes his campaign pitch.

"I certainly have to be impressed with your creativity in campaigning," one woman told Tucker recently after talking with him for a couple of minutes.

"I've gained a point or two, haven't I?" Tucker asked. The woman agreed that he had.

Tucker, like Barry and Washington, has criss-crossed the city in search of every voter that might make the difference between victory and defeat. He has campaigned before black groups and white gatherings and at the endless run of candidate forums; he has worked his way through the singles' crowds at the city's discos and talked with elderly voters who have been wheeled into recreation rooms to hear him; he has flown to Hot Springs, Va., to socialize with realtors and mortgage bankers, and driven to Bethesda to dine and chat with group of black professionals.

While Tucker obviously feels that the more people he can see in the next 3 1/2 weeks the better his chance of winning, his opponents feel the opposite is true.

Tucker's opponents, and even some of his supporters, regard him as the weakest of the three major mayoral candidates as a campaigner. Whether it is wishful thinking or not, some aides to both Barry and Washington say they feel that the more people see Tucker the less likely they are to vote for him.

Barry has compared Tucker to "an aging, Southern mansion, the closer you get the more you can see the chipped paint."

Earlier in the campaign, Tucker's aides exhorted him to show more emotion on the stump and not be so reluctant about shaking hands with voters. At some appearances, Tucker seemed to wait for people to come meet him.

That has largely changed and his supporters seem genuinely excited when he finishes his now familiar refrain in ringing tones: "Remember in this election, it's Tucker . . . Mayor . . . Now!" Still, on occasion, he seems to lapse into a professorial manner of speech that hardly excites his listeners.

"I'm not a standoffish person," Tucker says, "although some people perceive me that way. I've been so people-oriented all my life. I'm as comfortable at a 12th Place fish fry as I am in the White House."

Tucker has encountered one other problem in the last month - continual questions from voters about a U.S. Labor Department report alleging that city council workers hired under the federal Comprehensive Employment and Training Act (CETA) jobs program were picked more for their political connections or friendhsip with council members than for their work qualifications.

Tucker has denied any wrongdoing, but at the same time has moved to tighten hiring regulations for the local CETA program.

What Tucker is banking on is that his message will outweigh any shortcomings he might have as a campaigner. And that message is a simple one: The city government has been badly run by Walter Washington and Sterling Tucker is the candidate with the experience and credentials to make it work.

"The mayor has not been able to make the transition from the prehome rule government," Tucker told a small gathering of Northwest Washington residents at a recent wine-and-cheese party. "There's this lack of recognition of the transition. There's no ability to make the transition.

"The problems are getting ahead of us, but we can do something," Tucker said as he gestured with both hands to emphasize the point. "Do we begin to take charge or are we still a ward of the federal government? I'm going to lead."

Tucker says Washington is his chief opponent and thus seems to launch a verbal assault on Barry only after Barry has attacked him.

"His credentials just don't recommend him," Tucker says of Barry. "His presidency of the school board was one of the more chaotic periods. He hasn't demonstrated leadership."

What Tucker says chiefly qualifies him to be mayor are 18 years he spent as executive director of the Washington Urban League and his 1970s' tenure as the appointed council vice chairman and then the elected chairman. The Urban League here and nationally was generally regarded as one of the more conservative civil rights organizations, but the local group under Tucker played an important role in opening the door to thousands of jobs for blacks.

As for his performance as council chairman, Tucker regularly tells prospective voters that "under my leadership" Congress has not vetoed a single council-passed bill.

"I've been trying to protect a vision of the city, what can happen, how we can affect our destiny, how services can be delivered, how government can really work, some sense of the city's greatness," Tucker says.

As part of that effort, Tucker's campaign has produced a string of position papers on various issues, but in the end Tucker tells voters the whole election boils down to a question of leadship. "We all want better housing, more jobs and the like," he told one group.

Tucker also plans to strengthen his campaign through extensive appearances by one of his staunchest supporters, Del. Walter E. Fauntroy (D-D.C.). Fauntroy, who has campaigned some for Tucker already, likes to tell voters that he needs Tucker elected in order to create a "partnership for change" in the city, a relationship he says has been non-existent while he has been in Congress and Walter Washington in the mayor's office.

Tucker says he finds a "general acceptance of me across the city. My guess is tht mine is the broadest support." He concedes that the city's labor leadership is mostly behind Washington, but says that rank-and-file workers are often for him. He also lays claim to the largest share of black professional people.

It was such a group that gathered one night in the $200,000 Bethesda home of Dickie S. Carter, president of the Minority Contractors Assistance Projects.

"I see dollar signs when I see you guys," Tucker joked as he was introduced to the 13 black doctors, dentists, contractors and developers. The meeting had been set up by Carter as a way of trying to enhance the opportunities for black businessmen in the District should Tucker get elected. Tucker also saw it as a way to raise some more money.

So Tucker sipped a Scotch on the rocks served by a white bulter and then dined on roast beef and pork, a lettuce and mushroom salad, a rice, chicken and shrimp dish and a can of Coors beer.

He talked with the businessmen for 1 1/2 hours, until just after midnight, and assured them in various ways that he, too, was concerned about black entrepreneurs getting their fair share of the District government's business.

"I want to run a government with men like you," he told them as he concluded the session in the Carters' elegant wood-and-glass family room with its high-pitched cathedral ceiling. But in his next breath Tucker was back to the reason he drove to Bethesda.

"Now if you'll just give me the money I'll be going," he joked.

Carter handed him a fistful of checks and the take was another $1,300 toward what Tucker says will be a $300,000-plus campaign by election day.

Tucker and his aides say they think he is strongest in Ward 4 and 5 in Upper Northwest and Northeast Washington and Ward 7 (parts of far Northeast and Southeast Washington) that compose a large part of Washington's black middle class. He says he thinks he trails Barry in largely white Ward 3 west of Rock Creek Park and possibly in Ward 6, which includes Capitol Hill and part of Anacostia. He said he believes he is in a close race with either Washington or Barry or both in the city's three other wards.

As the campaign draws to a close, Tucker and his aides say he will continue to campaign much as he has since early in the year, running from one campaign appearance to another with the claim that he has the best credentials to the city.