WHEN MARION Barry's successor is sworn in next Jan. 2, the change in leadership of this city's government will go well beyond the mayor's office. Even without knowing the full field of candidates for D.C. Council, you can expect marked differences in the makeup, influence and direction of the city's legislature -- the 13 men and women who enact local laws. And what these members wind up doing -- how they coalesce or split on issues -- could take on a whole new dimension as they do business with a new mayor.

Whatever else one may know or guess about the next mayor, it will be difficult to top the crafty influence Mayor Barry managed to have with the council. Though his sway with the lawmakers has been disintegrating as his troubles have mounted, Mr. Barry has been able over the years to win over enough members at least to reflect his points of view on major issues and often to carry the day. Though three of the candidates for mayor are members of the council -- Chairman David Clarke, at-large member John Ray and Ward 4's Charlene Drew Jarvis -- not one of these three as mayor would be likely to carry as much weight with the next council as Mr. Barry used to.

Regardless of any other election results, the strong likelihood that John Wilson, now a Ward 2 council member, will take over as chairman should create a stronger legislative presence in the government. Mr. Wilson knows the District Building inside out and has strong, informed opinions about what could be done to clean house and save money. On the council, there are two at-large seats up for election this year too. One is being vacated by Betty Ann Kane, who is running for the delegate seat of Walter Fauntroy, now a candidate for mayor. The second is held by Hilda Mason of the Statehood Party, who is seeking reelection and who may face strong competition in the second round, when independents will join the Democratic and Republican nominees to compete for the two at-large seats.

There are four ward seats up for reelection too. In Ward 1, incumbent Frank Smith will seek another term. In Ward 3, incumbent Jim Nathanson is expected to win in the primary, but will face at least one lively challenge in the next round from independent Jim Kalish. In Ward 5, incumbent Harry Thomas Sr. is running strong so far. In Ward 6, four-term incumbent Nadine Winter is under strong challenge by Harold Brazil, a Pepco lobbyist, as well as a number of other candidates.

In all, there are more than 30 candidates who have gathered petitions or filed with the D.C. campaign office for the seven council seats. If Mr. Wilson wins his election for chairman, that will open up his Ward 2 seat. Two more "ifs": if Mr. Ray wins for mayor, his at-large seat will be open, and similarly if Mrs. Jarvis wins for mayor, her Ward 4 membership will open up. Add to all this the shifts in chairmanships of key council committees -- including Finance and Revenue, headed by Mr. Wilson, and Government Operations, headed by Mrs. Kane -- and you're talking new opportunities for the moving, or blocking, of local legislation.

The only way to know what any of this will mean in terms of substance -- of actual policies and directions at city hall -- will be to watch these contests closely for clues, as we will be doing for the duration. What is already becoming clear is that these council elections are a good bet to be the most important since the council came into being.