Weariness has settled over the long contest for mayor of the District of Columbia.

For nearly two years, Democrats have bumped and nudged each other for the inside track on endorsements and campaign financiers, and then gone head-to-head in an intense and sometimes bitter September primary from which some have still to recover.

The once touted campaign of Republican Arthur Fletcher, which has been expected to create the first real challenge to a Democratic monopoly of home-rule District politics, has failed to attract major endorsements, contributions, issues and from many indications, supporters.

Indeed, instead of the down-to-the-wire excitement that everyone anticipated and tasted in the Sept. 12 Democratic primary, there is a prevailing mood of fatigue. And it shows in the candidates themselves.

Republican Fletcher talks often of how refreshing the end of the campaign will be - win, lose or show. Democrat Marion Barry, sitting last week in still another waiting room in preparation for still another television debate, conceded that after two years of active campaigning, he is tired, too.

The candidates' forums that were a major battleground in the primary are held less often, now, and fewer people - including candidates - attend. Gone are the television ads and most of the multicolored posters, bumper stickers and buttons that sought to create political excitement.

No matter how "impressive" a candidate Fletcher is, and no matter how leery some are of a person with Barry's militant background sitting in the mayor's chair, the District of Columbia is a Democratic town. Many believe that the real voting and the real campaign ended Sept. 12.

"I'm not going out and work all day for Marion Barry like I did in the primary." one active Barry supporter said the other night at a non-political cocktail party. "I'm going to the polls, cast my ballot and go back to work."

The general election does have an element of being a referendum on Barry, the 42-year-old former codirector of Pride Inc. ex-president of the D.C. Board of Education and twice-elected at-large member of the City Council.

Barry, running an unconventional campaign that assembled a self-described motley crew of supporters and surged to victory in the final days of the race, won the Democratic primary with a narrow 35 percent of the vote.

The challenge to him has been to regroup enough of the city's Democrats, who make up 80 percent of the registered voters, to gain the victory that is usually assumed for the Democrat nominee. Fletcher's counterchallenge has been to show Democrats, whose crossover votes are essential to his victory, that Barry is not deserving of such support.

As the campaign moves toward a conclusion on Tuesday - or perhaps, sometime thereafter, depending on whether the city's hapless Board of Elections finishes counting on Tuesday night or some days thereafter - Barry's image still remains a problem, even in the eyes of some of Barry's staunchest supporters.

"I don't think a lot of people are ready for Marion Barry at his point," one Barry stalwart said privately yesterday. "They still see Marion Barry in the 60's as an activist. They see him as bring about to drastic a change. That will plead people to either stay home or to vote for the Republican."

Ivanhoe Donaldson, Barry's campaign manager, has consistently talked about the problems of being a front-runner. One of the principal hurdles that will pose for Barry on election day is overcoming apathy and overconfidence.

In 1974, when the city held its first election for mayor and City Council in more than 100 years, more voters turned out for the general election than for the primary, even though Democratic nominee Walter E. Washington faced only token opposition.

Several leading Democrats - Donaldson is an exception - think the reverse will occur Tuesday, mainly because there are fewer interesting city Council races this year than in 1974.

Donaldson expects about 100,000 - slightly more than the 98,000 who voted in the 1974 general election.

John Wilks, Fletcher's campaign chairman, expects voter turnout to be as low as 70,000 or less. And, he said, because Republicans tend to be more hard-core than Democrats and a higher turnout would probably bring out more "soft" Barry supporters, a low voter turnout is likely to benefit Fletcher.

Lilian Adkins Sedgwick, vice chairman of the D.C. Democratic State Committee, said the experimentation with a punch-card voting process in Ward 5 may cause delays and also encourage some people from going to the polls in that usually vote-rich ward.

Warren Graves, an active Democrat who ran Mayor Walter E. Washington's unsuccessful primary campaign, said the 12-day vote count delay that followed the primary may have also contributed to no-shows at the polls on Tuesday.

Susan Pennington of the U.S. Labor Party and Glova E. Scott, a member of the Socialist Workers Party who is running as an independent, also are in the race for mayor. But the contest has been primarily between Fletcher and Barry.

In the seven races for City Council as well as that for delegate to Congress, non-Democrats appear to have made few inroads into the ranks of Democratic voters, thus making tre general election contests far less competitive than those in the primary.

The closest races appear likely to come in wards 5 and 6, but only because in those instances Democrats who narrowly lost in the primary are mounting write-in campaigns against the incumbent victors. Patricia Rice Press is challenging incumbent Nardine P. Winter again in Ward 6, while Robert Artist has launched a similar campaign against incumbent William R. Spaulding in Ward 5.

Statehood Party member Hilda Mason, the only non-Democrat up for reelection, is among the favorites in a field of four candidates vying for two at-large City Council seats. There are also council contests in wards 1 and 3.