If you want to maintain good relations with labor unions, you don't propose to freeze their pay and cut their health benefits, and you don't toast their health with Coors Beer, the subject of an AFL-CIO boycott because of alleged antiunion activities.

But Mayor Marion Barry has done all that recently, and his actions have damaged his recent efforts to cultivate better relations with organized labor, a large and sometimes powerful voting bloc in the District.

Barry's hard-line approach in the current negotiations with seven unions representing 18,000 of the city's 30,000 workers has earned him some hard feelings among labor officials, who have previously been supporters but whose enthusiasm for the mayor may wane if the contract impasse persists.

The District's three-year labor contracts for municipal workers, police and firefighters expired Sept. 30 and negotiations continue, with the key issues involving Barry's effort to freeze pay scales in the first year, eliminate city payments of dental and optical premiums, and reduce sick leave.

Last time around, in the election year of 1982, city employe unions received pay raises totaling more than 20 percent in their last contract and supported the mayor's campaign. But now they are crying foul at the District Building's insistence that the city must hold the line on wages and cut back on benefits.

Barry compounded his labor-relations problem last Saturday night when he allowed the Adolph Coors Co. -- regarded by the AFL-CIO as one of the most antiunion employers in the nation -- to finance and sponsor a reception that Barry gave for members of the Congressional Black Caucus.

The Colorado-based Coors Co., the nation's fifth-largest brewer, is the subject of a seven-year boycott by the AFL-CIO, which has accused the firm of exploiting its employes, particularly women and minorities. The unions charge that Coors has used discriminatory employment practices and violated workers' rights by the use of mandatory lie-detector tests, search and seizure of employe belongings and other practices -- allegations denied by Coors.

Coors has vigorously countered the boycott and negative publicity with an advertising and public relations effort that focuses on its expanded opportunities for minorities. As part of that, the company sponsors social and political functions such as Barry's reception at the Washington Hilton Hotel.

Barry has worked to cultivate good relations with labor since his last election, when a number of unions supported his opponents. He has since opened an office of labor liaison. The labor office is run by Gwen Hemphill, who worked in the White House under Jimmy Carter and is well-connected to District unionists, and Jane Danowitz, a former lawyer with the American Federation of Government Employees.

"The mayor is very pro-union and the mayor has taken care of public employes," Hemphill said. "The mayor has not riffed any employes and that is one of the most important issues. The mayor is not unfair, nor is he insensitive" to the needs of labor. Hemphill was referring to the fact that Barry has not laid off employes under the current contract, although there were RIFs in the financial crisis of his first administration.

Hemphill said she thinks criticisms about the negotiations will quickly blow over. "The mayor has to deal with certain realities. When we tried to explain to some of the rank-and-file that due to budget restrictions, they will not be able to get lucrative pay raises, it is difficult," she said.

Joslyn N. Williams, president of the Metropolitan Washington Council of the AFL-CIO, which represents 150,000 workers, said that Barry is reacting this year partly to "the perception that there was a sweetheart deal in 1982" in which the mayor gave fat raises to the unions in return for political backing. True or false, Williams said, that perception has caused both Barry and the unions to project a tougher image to show they are not in cahoots.

"I suspect the mayor is trying to impress the taxpayers that he is a tough chief administrator," Williams said. "Unfortunately that image is portrayed at the expense of city workers. I don't think the mayor is antiunion. But he is an employer, and we must not lose sight of the fact that, in the final analysis, he is an employer. And that is how it is supposed to be," he said.