Three Washington area labor groups, in the first key labor endorsement of the Senate race in Maryland, announced yesterday their support for Rep. Michael D. Barnes (D-Md.), placing in his camp several of the state's most active and well-organized union locals that claim about 80,000 members.

The early endorsement could prove particularly helpful in Prince George's County, where many union members live but which has produced no favorite son candidate to lock up support for the Senate race.

The announcement, however, drew sharp criticism from a prominent Baltimore labor leader, who called it premature. Henry Koellein, president of the Baltimore Labor Council, said Baltimore labor groups almost certainly will support their own local favorite, Democratic Rep. Barbara Mikulski, confirming that Maryland's labor leaders, united only a month ago in support of gubernatorial candidate Stephen H. Sachs, will be dividing their resources among the U.S. Senate contenders.

"We've urged all of them to wait for the state convention. We would rather them wait, to have a consensus vote," said Edward Lamon, president of the Maryland State-D.C. AFL -- CIO, which claims 400,000 members in the state. "It didn't happen."

Lamon noted that each of the four expected Democratic contenders -- Barnes, Mikulski, Gov. Harry Hughes and Baltimore County Executive Donald P. Hutchinson -- has been elected with varying degrees of labor support in the past, and thus a united labor effort behind one candidate is unlikely.

"With friends running . . . I don't see any candidate getting two-thirds of the unions to support them," he said, referring to the minimum required to win endorsement of the state-wide umbrella.

Yesterday's endorsement came amid a perceptibly heightened pace of political activity among the contenders, including accelerated fundraising and publicity-grabbing campaign stops around the state. Barnes has begun an aggressive fund-raising drive in Washington and around the country, with several events scheduled in New York City near the Thanksgiving holiday. Today he will be passing out balloons and literature at Memorial Stadium before the University of Maryland football game.

Next week Hutchinson has scheduled another in a series of day-long campaign trips to the Eastern Shore, and is planning a major midwinter fund-raiser. Hughes has begun a round of private, "upscale" fund-raising dinners, according to his treasurer, Fred Wineland, as has Mikulski, who held her third fund raiser earlier this month, a spokesman said. Mikulski also has stepped up her schedule of speeches and meetings with activists around the state, the spokesman said.

Barnes, a liberal Montgomery County lawyer first elected to Congress as the underdog in 1978, enthusiastically thanked the labor representatives for their support, saying "This is a tremendous boost at the early stage of the campaign."

"When we've won a year from now and we're celebrating down in the United States Senate, gentlemen, I will remember. I will remember that you were there when it wasn't easy," he said at a gathering at one local's headquarters in New Carrollton.

The three umbrella groups are the Food and Allied Service Trade Union; and The Greater Washington Building Trades Unions, both AFL-CIO affiliates; and the Teamsters Joint Council. The three have a combined membership of 70,000 to 80,000, according to their presidents, including the United Food and Commercial Workers Local 400, one of the area's largest union locals with a membership of about 30,000.

Thomas McNutt, president of UFCW Local 400, said he and other labor leaders, not Barnes, engineered the early endorsement. He said he hopes to unite labor groups from around the state behind Barnes' candidacy, and although he considers Mikulski strongly pro-labor, his group rallied behind Barnes as the more viable candidate.

"You have to make a choice about who can be effective and who can win. We personally believe that Mike ultimately would make a better candidate and would have a better chance of winning," said McNutt. He added later that Mikulski "through her vigorous representation of her constituents" could be perceived as "very parochial" and less attractive to voters outside Baltimore.

McNutt's group, however, was roundly criticized by Koellein. "I am just flabbergasted that they would come out like this," he said. ". . . The tail don't wag the dog and in this case you're talking to the tail. I think it's a weakness on labor's part that we're going to be split like this. . . . The most any of them should be doing is getting together to make a recommendation to the state AFL-CIO."

As to who Koellein's people would support, he said flatly, "Barbara Mikulski is the greatest congressperson ever from Maryland and the Baltimore groups are going to be leaning toward Mikulski."

Neither group said it seriously considered supporting Hughes, the only candidate who has not yet formally announced.