Arthur A. Fletcher's strategy for defeating Marion Barry in next week's election for mayor of the District of Columbia is not particularly new, because is some respects it is the same strategy Barry used to defeat his two opponents in the Democratic primary only seven weeks ago.

Fletcher looks at Democrat Barry's suddenly fatter campaign coffers and labels him the "big money" candidate. He sees Barry's newly obtained endorsements and says Barry does not represent the people. He hears Barry talk of being familiar with city problems and then tells voters that that is because Barry is "part of the problem."

And with only differences on a few moral issues and variation of style separating his campaign pitch from that of Barry, Republican Fletcher talks about Barry's history as one of "irresponsibility" and "distrust" in the same way Barry cited Washington's administration as one of "bumbling and bungling."

Relying heavily on television appearances and anti-Barry sentiments, Fletcher is hoping to put together a spontaneous coalition of disgruntled Democrats, moral conservatives, hopeful Republicans and those willing to try something entirely new to defeat front-runner Barry.

"There are the makings of an underground "Stop Barry" movement within the Democrat community and if I were running as an independent, it would make it much easier for that movement to surface," Fletcher said last week.

"There is a considerable (mental) block to voting for a Republican in this city in spite of the negative image Mr. Barry manifests. They're not going to vote for me because of my Republican tag. They've got to be reminded of what they're about to do (if they vote for Barry)."

In a city where eight of every 10 registered voters are Democrats, the Republican nominee could ordinarily be written off. But with the rules of city politics still unwritten - this is only the second general election for mayor in more than a century - and with Barry coming out of the primary with only 35 percent of the vote, Barry strategists are not willing to count Fletcher out.

Instead, they have adopted a campaign approach very similar to that of mayor Walter E. Washington whom Barry defeated. It emphasizes Barry's record, streses his experience and instead of talking about an "avalanche victory" as he did earlier in the general election campaign, Barry now talks modestly about winning "51 percent of the vote."

And, with the exception of a new backhanded counteroffensive against Fletcher's campaign claims, it is now Barry who has become the statesman on the stump, winning praise from some of those at Shiloh yesterday because, they said, he carried himself "properly" and would provide the city with "the right image" on Capitol Hill.

"We don't need on-the-job training (for the mayor), not this year, not this year," Barry told an upper Northwest Washington civic league meeting last week. It was almost the same line that Mayor Walter E. Washington had used to put down Barry's candidacy in the Democratic primary.

Fletcher, a 53-year-old labor consultant who served as assistant secretary of labor for wage and employment standards in the Nixon administration, is running for office in the District of Columbia for the first time and, he often says, making for this city a real general election, contest for the first time, too.

"If I weren't running for mayor, Marion Barry wouldn't be here," he told the Civic League of North Portal Estates last week.

Fletcher's strategy centers on 63 of the city's 137 precincts, precincts in which, Fletcher strategists noted, Barry ran poorest in the Democratic primary. These precincts are mainly in wards 4 (upper Northwest Washington east of Rock Creed Park), 5 and 7 (Northeast and Southeast Washington) and 8 (far Southeast and Anacostia). Barry ran third in all those wards.

Fletcher said his organization's canvassing has found a significant number of undecided or uncommitted voters in those areas. "The outcome of the election is in the balance," Fletcher concluded.

Fletcher said he is also finding some weaknesses in the four wards that Barry carried - Ward 1 (Adams-Morgan, Mount Pleasant and Cardozo) Ward 2 (Dupont Circle, Foggy Bottom and new Southwest) Ward 3 (west of Rock Creek Park) and Ward 6 (Capitol Hill, near Northeast and parts of Anacostia).

Of this group, Ward 3 is pivotal because nearly half the city's 22,000 Republicans live there and make up 24 percent of the ward's voters. Fletcher said he expects to get 7,000 votes out of Ward 3, which he says should equal the number of votes Barry will receive. (Barry received nearly 6,000 in the primary).

To that base, Fletcher intends to add 40 to 55 pecent of the Democratic vote in wards 4, 5, 7 and 8 to defeat Barry.

Fletcher said in an interview last week that the local Republican Party lacks the experience and track record to have developed the kind of sophisticated and effective organization that many District politicians consider all-important in winning an election.

Thus, he said, on Nov. 7, his campaign will have "minimal capability of turning out votes" on election day.

Rather, he said, he will rely on "concerned citizens voting against Mr. Barry and his image and going to the polls in spite of rather than because somebody came to take them."

As proof of his allegation that Barry is "irresponsible" and not to be trusted, Fletcher accused Barry of "mismanaging" the self-help group Pride Inc., and of being responsible for the current failings of the city's school system (Barry was school board president for two years). Fletcher also said Barry's support of marijuana decriminalization and gay rights would create a morally unhealthy environment for young children in the city.

By inference, Fletcher also has begun to kindle memories of the belief some older Washingtonians held that Barry and his "street dudes" at Pride were "hoodlums" who were reponsible for the burning of the city during the 1968 riots.

For example, Fletcher said at one point last week that some people are afraid to announce their support for him publicly because they fear "retaliation" from "vindictive" Democrats.

The Fletcher campaign is hoping, with no certainty, to have some radio advertising this week. Otherwise, its low-budget operation - Flethcer does not expect to raise $50,000 and has raised only $41,000 so far - has offered no possibility of any media at all.

If Barry is a new candidate because of his front-running status, so too is his campaign strategy. In the primary election, Barry targeted wards 1, 2, 3 and 6, won those wards and finished a respectable third elsewhere to piece together a victory.

But in this election, Barry's battle plan is to win all eight wards, and a key to that strategy is making peace with former Washington and Tucker supporters.

Beginning this week, the voices of Tucker and Washington are to be heard endorsing Barry on radio commercials and that is expected by Barry strategists to shore up his support among some Democrats.

The Barry organization took a poll early this month that found he was beginning to pick up more support since winning the primary. That momentum has been reinforced by endorsements he has received for many of the businessmen, labor leaders, and churchmen who supported Tucker and Washington during the primary.

But, one Barry strategist said privately, the poll also showed that some persons - mainly those 50 years or older, black and Washington supporters - were still leery of having Barry as mayor.

The Barry organization already has its telephone bank in regular operation, Donaldson said, and for weeks has been working to meld together the hard-core army of Barry volunteers from the primary with campaign workers from the Tucker and Washington organizations.

Ivanhoe Donaldson, Barry's campaign manager, said the burden of winning the election is actually on Fletcher, who must do more than just campaign against Barry. "Fletcher has to show them a reason why they should go with him and I don't think he can do it," Donaldson said.

Usually in Washington, with its high Democratic registration, winning the primary contest is tantamount to winning the election. In this case it is different, Donaldson said, and the worst that could happen is that Democrats could become overconfident.

"How do you generate excitement? That's a hard question," Donaldson said last week. "I think the voters are pretty much prepared to go with Marion Barry. But when you don't have a strong gut reaction (against anyone) and voters are pretty much satisfied with the candidates in the field, it tends to generate apathy."