With endorsements reflecting organized labor's split in the race for D.C. mayor, the largest union of District government workers threw its support yesterday to candidate Walter E. Fauntroy, while a large building trades union will announce today it is backing John Ray.

William Lucy, the international secretary-treasurer of the American Federation of State, County and Municipal Employees, announced the union's endorsement of Fauntroy at an afternoon news conference that gave the D.C. delegate another opportunity to attack Ray, a rival in the Democratic mayoral primary, for accepting several large contributions from developers.

John P. O'Connor, secretary-treasurer of the Washington Building and Construction Trades Council, said his union will endorse Ray at a news conference this morning. One-third of the construction union's 30,000 members live in the District, O'Connor said.

Meanwhile, in an unrelated political development, Mayor Marion Barry said he was undecided about seeking an at-large seat on the D.C. Council this fall, depending on the verdict in his federal trial on 14 drug and perjury charges.

"I've enjoyed serving this city and I would like to continue," Barry said during an appearance on radio station WOL-AM (1450). But he also indicated a preference for something outside politics, saying, "I'm going to try to probably use my talent and skills doing something else -- I'm not sure exactly what -- to help people."

"I've not yet decided whether or not to run for city council," Barry said. "I'm trying to be pretty introspective about it."

In a separate interview with WETA-TV (Channel 26), a portion of which was released yesterday by the station, Barry said he was now inclined to support a two-term limit on the city's key elective offices.

"I was opposed to that initially," said Barry, who had planned to run for a fourth term before his Jan. 18 drug arrest. "But because we don't have much room at the top, I think I'm coming to the conclusion we ought to have two terms for mayor, and for city council, and for school board, and for chair of city council -- so at least we begin to get some movement."

With 40 days left before the Democratic mayoral primary, Fauntroy continued his attack on developer contributions to Ray, saying, "In John Ray's Washington, it's the out-of-town developers who call the shots . . . . In John Ray's Washington, the fast-buck artist and the quick-change politician have worked hand in hand -- raising rents, dashing dreams and ripping communities apart."

Ray's spokeswoman, Margaret Gentry, declined to respond to Fauntroy yesterday.

Lucy said AFSCME Council 20, which represents 8,700 D.C. government workers, endorsed Fauntroy on the strength of his nearly 20-year record as the city's nonvoting delegate to Congress. Fauntroy said the group's endorsement in the 1971 primary election for delegate was a turning point in that campaign.

However, this year, the support of organized labor is split between several of the Democratic candidates. For example, while Fauntroy won the endorsement of parent Council 20, some smaller AFSCME locals are supporting candidates Sharon Pratt Dixon, a lawyer, and Charlene Drew Jarvis, a D.C. Council member from Ward 4.

Similarly, the parent board of the Teamsters in Washington split its endorsement between Fauntroy and Jarvis, while a major Teamsters local is helping Ray in the race.

Bernard Demczuk, a longtime local labor organizer who is Jesse L. Jackson's chief aide on issues affecting that community, said Barry's absence from the mayoral field freed the unions that had supported him over the years to coalesce behind other candidates. D.C. Council Chairman David A. Clarke is widely viewed as having a strong pro-labor record, but has not captured many significant union endorsements.

Demczuk said that far from diluting labor's strength over time, the current fragmentation of union support could help workers, in part because the next mayor will still have to negotiate with District unions. This year's political dynamic "has split us up but, paradoxically, it's made us strong," Demczuk said.

"Labor is being very smart in not endorsing one single candidate, because this thing may go down to the wire," Demczuk added. "By spreading its support among several candidates, what labor has done has been very positive."

O'Connor of the building trades said the split in organized labor reflected the electorate itself.

"If the labor community has come together and supported one candidate, that person would have won the primary in a landslide," O'Connor said. "It's chopped up right now for the primary season, but whoever wins this thing will need labor's support to run again" in four years.