The Maryland and D.C. chapter of the AFL-CIO, long considered a potent force in Maryland politics, is so divided that it will not endorse any of the three Democratic candidates for U.S. Senate, all of whom sought the united backing of organized labor.

The lack of an endorsement resulted because none of the candidates -- Rep. Barbara A. Mikulski, Rep. Michael D. Barnes or Gov. Harry Hughes -- could muster a two-thirds vote of the 500 delegates attending an endorsement convention in Baltimore Thursday night.

The lack of an endorsement means that the statewide labor organization will not send a mailing on behalf of any candidate to the state's 425,000 AFL-CIO households before the Sept. 9 primary.

"You have three good friends of labor running," said Edward R. Lamon, president of the organization.

Statewide labor leaders said they were seeking a candidate who would support legislation on job training, employe benefits and protections. They said all three had excellent positions on those issues.

The state AFL-CIO's decision to sit on the sidelines means that 400 local unions will be free to back their preferred candidates with mailings, phone banks and contributions of up to $5,000 each.

Sharp regional rivalries have sprung up between the Baltimore area groups supporting Mikulski, who represents part of the city, and the Washington area groups supporting Barnes, who is from Montgomery County.

The state labor organization has endorsed Attorney General Stephen H. Sachs for the Democratic gubernatorial nomination against Baltimore Mayor William Donald Schaefer, and is backing Deputy Attorney General Eleanor Carey. She is running for attorney general in the Democratic primary against Lt. Gov. J. Joseph Curran Jr. and former federal prosecutor Russell T. (Tim) Baker.

There is a general feeling among Democratic campaign strategists that a strong labor backing is usually more important in a Democratic primary than in a general election, because more union members are registered Democrats. One in five Democratic households has a member of organized labor.

Labor was credited with Democratic candidate Walter Mondale's victory in the presidential primary in 1984, although it failed to deliver the heavily Democratic state in the general election race against President Reagan.

In 1976, labor's backing, which translated into money, phone banks, and workers in the final weeks of the campaign, helped underdog Paul S. Sarbanes defeat former senator Joseph E. Tydings in the Democratic primary.

"Labor made a significant difference in that race," said John W. Eddinger, who worked for Tydings and now is campaign director for Hughes.

Lamon said that the state AFL-CIO will start operating phone banks later this month for Sachs and has done one statewide mailing on his behalf.

Carey, an underdog in her bid to become attorney general, called the AFL-CIO backing "the most important endorsement" she will receive in the race.

But not every candidate and strategist placed as high a value on the endorsement of organized labor.

"This is not Detroit," said one Democratic campaign manager, adding that in a Democratic primary it helps to have organized labor on your side but that an endorsement hardly guarantees victory.

In the Democratic gubernatorial primary in 1978, former Baltimore county executive Theodore Venetoulis received the labor endorsement, but Hughes swept to a surprising victory and Venetoulis came in third.

Some Democrats said that labor is becoming less significant in primaries because the rank and file often ignores the recommendations of its leadership.

Furthermore, some strategists said that in an era of expensive media campaigns, labor is less effective than television at mobilizing voters.

"We can't compete with a million-dollar media budget," said Thomas McNutt, president of the United Food and Commercial Workers Local 400, which is backing Barnes. But, he added, "In primaries and state elections that don't get that sort of media attention, people handing out leaflets or making telephone calls do make a difference."

Some Democrats also said that labor is more effective in local races that rely less on media and more on manpower.

"They give money, and they give workers. Not many endorsements do that," said Del. Wendell H. Phillips, a Democrat running for the House in the 7th Congressional District in Baltimore. He considered it a victory when he stopped the state AFL-CIO Thursday night from accepting its executive board's recommendation to endorse his Democratic opponent, state Sen. Clarence M. Mitchell.