Mayor Marion Barry, former City Council chairman Sterling Tucker and former mayor Walter E. Washington could hardly have split the vote more closely in the 1978 Democratic primary. Washington got about 32 percent, Tucker about 33 percent and Barry, whom few expected to win, became mayor with the underwhelming mandate of just over 34 percent of the city's Democratic voters.

And now, the unannounced candidates for mayor in next year's election are developing their scenarios from a premise: That there is likely to be a field of three or more candidates in the Democratic primary. Once again the city's mayor would effectively be chosen by a minority of D.C. Democrats.

In such a single-party town, the Democratic primary in September is the ball game. And since there is no provision for a runoff, the Democratic primary winner becomes the odds-on favorite to crush the Republican nominee in the November general election. Indeed, Barry ran up a huge majority in the general balloting over his Republican opponent.

The idea of a primary runoff has surfaced several times over the years. It is in the air again, courtesy of legislation proposed by City Council Chairman Arrington L. Dixon.

But few people expect that bill to get far. Half the City Council wants to run for either mayor or council chairman, and such a clutch of candidates is unlikely to agree on anything, much less a change in the way the mayor is elected. Especially since these are the prospective candidates who hope to slip through by plurality.

In the climate, the proposal is languishing both inside and out the District Building. Said former D.C. Democratic Party chairman. Robert B. Washington Jr.,: "On the day after the election, I think I would support a runoff."

Dixon said in a recent interview that he thought his bill, which would require runoffs in any city election in which no candidate received a majority, would ensure that winning candidates "get a greater mandate" from D.C. voters. Citizens, he said, would be able to make sharper Choices. Their officials would not face the kind of fractured electorate Barry has to contend with.

But decisions about whether to have runoff elections, like most other decisions about politics, are made with specific political ends in mind. San Francisco established a runoff 10 years ago, for example. The conventional wisdom is that the action was taken to block a popular black state assemblyman, Willie Brown, from consolidating the black vote and becoming mayor with about a third of the votes cast. For years, the runoff provision was called the "Willie Browm Law."

In the early days of home rule, a runoff provision wasn't given much consideration. "I don't recall any serious thought," said former Mayor Washington. "I think everybody was just glad to at least get a provision in the charter for an election."

But as the city's politics evolve, so do its situationed ethics. Dixon tried before to get his runoff provision passed, but the attempt did not make it out of the Committee on Government Operations, headed by Council Member William R. Spaulding (D-Ward 5). One District Building source recalled that both Spaulding's council victories have been slim ones in crowded fields. Dixon's current bill is also in Spaulding's committee.

And there is the question of race, forever intertwined with Washington politics. A Runoff might dilute the black vote, the theory goes. The fear is that only the most faithful voters will troop to the polls for a runoff, possibly leaving out many blacks and giving areas like predominately white Ward 3, whose residents vote religiously, a disproportionate say in the city's politics.

And finally there are the publicly stated reasons for not having a runoff: To have a second election is too costly, and in other jurisdictions has proved difficult to administer.

"In my view, it is not a good idea" to have runoffs, said Democratic National Committee member John R. Hechinger. "I believe that if a person wins by 2 percent or so in a field of, say, 10 candidates, he's won sufficiently."

Which means that barring an unforseen confluence of public opinion behind some candidate between now and September, the next mayor won't be elected by the majority.