Former D.C. City Council chairman Sterling Tucker, who ran unsuccessfully for mayor in 1978 and abandoned a second try for that office earlier this year, announced yesterday that he will run for chairman of the City Council in this fall's elections.

Tucker's decision came as a surprise in political circles, where he had been expected to try for an at-large council seat. It had the potential of changing dramatically what had been a two-way race between incumbent Chairman Arrington Dixon and council member David A. Clarke (D-Ward 1).

Tucker, who was elected chairman of the council in 1974 and served until he gave up the post to make his 1978 mayoral bid, said he would challenge Dixon in the Sept. 14 Democratic primary because he has failed to provide leadership.

"I think the council itself is not the cohesive body it ought to be," said Tucker, who had endorsed Dixon in 1978.

He said some members on the council "expressed a desire for me to return." No council members have yet publicly announced support for any candidate for chairman.

Dixon issued a terse, two-sentence statement welcoming Tucker to the campaign.

Clarke reacted with criticism. "I think the city needs someone who really wants to be chairman," he said. "He Tucker wanted to be mayor. Then he wanted to be an at-large council member. . . . His third choice is the council chair."

Tucker, 58, enters the race with good name-recognition throughout the city--a fact borne out by early polls done while he was considering running for mayor. But he is starting late, after Dixon and Clarke have already laid the groundwork for their campaigns and after some key groups have already committed their support. The Metropolitan Washington Labor Council, for example, indicated some support for Tucker earlier this year, but decided to back Clarke instead.

Dixon has already raised $77,386 for his campaign, reports filed last week showed, while Clarke has raised $31,301.

Tucker, who said he hopes to raise about $200,000 for the race, acknowledged that he is starting late and that many of his former supporters are now backing other mayoral candidates or have signed up with Dixon or Clarke. But he said he felt he would gain support as the campaign went along.

"I'm starting from scratch. I am beginning to put an organization together," Tucker said yesterday.

Tucker said he decided against the at-large seat because "it was a mine field of potential conflicts of interest" with a consulting business he has been trying to establish.

He said his plan would have been to try to keep the consulting business as an at-large council member. As chairman--a job that pays $51,290, or $10,000 more than any other council seat--he would be barred by law from keeping any outside business. He said he would divest himself of the firm if elected.

Tucker's entry into the race was criticized yesterday by Joslyn Williams, head of the labor council, who said the unions "are irrevocably committed to Dave Clarke.

"His Tucker's decision was a mistake," Williams said. "It muddles the water and it won't change our position. I want to make that point as emphatically as I can."

Tucker, who has been largely uninvolved with local politics after his 1978 defeat, worked for a while as an assistant secretary of Housing and Urban Development under Patricia Roberts Harris, herself a candidate for mayor, before starting his consulting firm.