The D.C. City Council returned from its summer recess this week to an unfinished agenda containing enough politically charged issues to spark an early beginning to the city's 1986 political season, when the mayor, the council chairman and six council members will seek reelection.

District Building observers anticipate that the incumbents will use the issues to get their names and faces before the public and their constitutents as they deal with the cable TV franchise, the rent-control referendum and the city's prison crisis.

Those up for reelection in 1986 are City Council Chairman David A. Clarke and council members Frank Smith (D-Ward 1), Polly Shackleton (D-Ward 3), William Spaulding (D-Ward 5), Nadine P. Winter (D-Ward 6), Betty Ann Kane (D-At Large) and Hilda Mason (Statehood-At Large).

The council's first showdown will probably be over whether to allow the city's cable TV franchise holder to greatly modify its agreement with the city. This week, the council received additional information to justify the request of District Cablevision Inc. for modifications to help the cable firm obtain the necessary financing to build the system. But the nagging question is whether the firm, which was granted the city's cable franchise early this year, will seek to avoid wiring some areas of the city if it becomes too costly.

The modifications are a sticky matter for the council -- voting to revoke the franchise at this stage may make it appear that the council made a bad selection when it awarded the franchise. On the other hand, no council member wants constituents denied cable service or forced to wait a long time for it.

The rent-control referendum that will appear on the November ballot seeks to repeal major provisions of rent legislation the council adopted last April.

Two council members, Mason and Wilhelmina J. Rolark (D-Ward 8), worked with tenant activists to get the issue on the ballot, and at least one other council member, John Wilson (D-Ward 2), plans to campaign for the referendum's passage. The referendum also gives Chairman David A. Clarke, who has been silent about the referendum, the opportunity to resurrect a key issue in time for his campaign.

Tenant leaders viewed the rent legislation as pro-landlord and have threatened to defeat any council member who supported it. That raises the question of whether those members, Spaulding, Winter and Kane (D-At Large), will speak out against the referendum knowing that they could encounter opposition from tenant activists if they seek reelection next year.

Meanwhile, the council is expected to take a position on the prison issue. Although Mayor Marion Barry, Congress and the U.S. Attorney's office support the idea of a new prison, the council has taken no position. For the moment, it is closely watching the actions of a 15-member commission appointed by the council and the mayor.

Last month, the commission rejected a resolution calling for a new prison and adopted one favoring alternatives to incarceration. Some members have said they favor constructing a new prison. If that group prevails, the council and mayor could be forced to find a politically ideal place to put it.

The council's legislative agenda includes a number of issues affecting the city's business community. Included are a bill to allow the District to participate in regional interstate banking, major amendments to the city's no-fault insurance law and a bill to overhaul the government's control over its contracting authority.