Officials of the second largest union representing District of Columbia government employes yesterday endorsed Mayor Walter E. Washington for re-election and said they will provide free services and voluntary manpower for his campaign.

"We're committing the talents and energies of hundreds of dedicated men and women," said Geraldine P. Boykin, executive director of Council 20 of the American Federation of State, County and Municipal Employees. Council 20 is the union's umbrella organization representing 16 locals, the bargaining agents for 12,000 employes, half of whom work for the District of Columbia.

Boykin, in response to questions, said Council 20 did not have enough money to make a direct financial contribution to the mayor's campaign. But the council will be making an indirect contribution, she said, by paying for phone banks, providing volunteers to work the phones, passing out literature to their membership and delivering campaign volunteer manpower from nonteaching union members who work for the D.C. school system.

District of Columbia school board employees are the only civil servants in the city not covered by the Hatch Act's prohibitions against overt political activity in partisan political campaigns.

The union, which represents 3,000 nonteaching staff such as clerks, secretaries and teachers' aides, will draw upon "all our members in the school system" to work in the mayor's campaign, Boykin said.

The 5,000-strong Washington Teachers Union recently endorsed mayoral candidate Councilman Marion Barry and says there will be 500 teachers working in his campaign, according to William Simons, teachers union president.

Mayor Washington, following Council 20 president Leroy Joseph at the Northwest Rhode Island Avenue Executive House motor hotel press conference, said, "This union represents working people, the people who are undergirding the efforts of this city."

In a Washington Post poll conducted June 1 to 5, Mayor Washington was ahead of his rivals, Council Chairman Sterling Tucker and Councilman Marin Barry, among working class families with incomes of less than $18,000. Overall, Sterling Tucker was leading the mayor by a slight 24 to 20 percent margin with Barry trailing a close third with 18 percent. Thirty-five percent of those polled were still undecided, and the poll showed that the race is much in doubt.

In a related development, Ronald E. Hampton president of the small, 200-member Afro-American Police Officers, the bargaining representative for 3,700 of the police department's officers.

Moore "has been a consistent advocate and champion of the rights of grass roots citizens in" Washington, Hampton said at a District building press conference. Hampton, whose union membership, like all Washington police officers, cannot participate in political activities, said he was unsure how they would be able to help Moore.