The biggest union local in the Washington area is not holding a Labor Day celebration today. "Our members would rather work," says a smiling Tom McNutt, president of the United Food and Commercial Workers Local 400. "They get $27.88 an hour on Labor Day."

Not bad for grocery store clerks, McNutt says proudly of the holiday wages, which speak to the clout of Local 400 and its 27,000 members in Washington and the Virginia and Maryland suburbs. In an area where unions are not particularly strong, it has organized more than 90 percent of the retail food industry and has secured for its members some of the highest wages in the country, exceeded only by their counterparts in San Francisco, Chicago and Minneapolis-St. Paul, according to a spokesman for the international union.

The product of a recent merger of the retail clerks and meat cutters, it is the largest local in the largest union in the AFL-CIO, easily the biggest in the area. So, whether its clerks are behind food counters or at picnics, Local 400 would seem to have plenty to celebrate on Labor Day 1981.

But on that note, McNutt stops smiling. Even this potent union, whose glistening, stone-and-glass headquarters in Landover is almost as imposing as the Peoples Security Bank of Maryland building next door, sees this Labor Day as a dark one for the movement.

The shadow, he says, is not cast so much from within the retail food industry, where the economic problems include the recent closing of 54 Pantry Pride grocery stores in Washington, Maryland and Delaware; the first quarterly losses of Giant Food Stores Inc. in a decade; and the financial woes of A&P.

McNutt, like his counterparts in the more struggling industrial unions, says his worries on this Labor Day are focused largely on one man: Ronald Reagan, whose hard stand against the Professional Air Traffic Controllers Organization strike has been read by many labor leaders as the opening shot in a battle against their movement. A convention of carpenters and joiners recently erupted in applause at Democratic National Committee Chairman Charles Manatt's characterizations of the Reagan administration as a "wrecking crew" and "the most antiunion, antilabor administration in Washington since Calvin Coolidge."

McNutt even blames "this antiunion sentiment emanating from the White House" for the most recent challenge to his local by Giant Food Stores Inc. and the other leading food retailers. The stores, whose recent losses came largely from cost-slashing during the recent Washington-area food price war, asked the local's cashiers, meat cutters, frozen-food clerks and other food store employees to help make up the difference by forfeiting their scheduled pay increase -- scheduled, under the 1980 contract, to start on this Labor Day. Local 400, like two other unions to whom the stores made the request, "respectfully denied" it in a letter last week.

"It was just the craziest request I've ever received," said McNutt, who spent hours saying essentially that on talk shows as the issue became a media event. "I don't think that kind of request ever would have occurred to them the food stores under any other administration . . . They know there's no place we can go in the federal government and get any sympathy." Essentially the same lament is being heard throughout the labor movement.

"That's the same boilerplate stuff they pull out of the hat every time they the union get pushed into a corner," said Giant vice president Roger Olson, who stressed that he likes and respects McNutt nonetheless. "Here's the best managed food chain in the country, and we're going to lose $3.5 million. If we're in trouble, everyone's in trouble and let's do something before we turn into a Pantry Pride ."

But, regarding what he calls the "store wars," McNutt responds, "If they had doubled their profits, I don't think they would have asked if they could reopen the contract and double our increases." And he still blames the losses on the store's own decisions to fight the price wars, not on the union wages.

Store-wide wages for union employes average out to $8.70 an hour and will increase today to $9.40 an hour, Olson said. Experienced, full-time food store clerks have earned $8.94 an hour for the last year, or $357.60 for a 40-hour week; their pay goes up 75 cents an hour today to $9.69 an hour, or $381.60 a week, according to the contract. The Labor Day pay of $27.88 an hour is a combination of holiday pay plus double the hourly rate, McNutt said.

The 92 percent unionization of the retail food industry here, unusually high by Washington standards, is due largely to the dominance of major chains, which are traditionally unionized, over independent stores, which are not. Local 400, which has a budget of $5 million a year, has organized all but one of the 10 top grocery companies in the Washington area. That one -- Magruder's -- is an independent. "We're working on him," McNutt says.

The local has expanded well beyond the food industry, partly to hold onto its strength as automation claims some of the jobs in retail food stores. The diversification parallels the movement by major industrial unions into white-collar and service jobs. Recently, Local 400 has expanded into health care, winning an election at the Wisconsin Avenue Nursing Home and mounting an organizing drive at Southern Maryland Hospital Center in Clinton. In the banking industry, another of its targets, the local recently organized the small Hemisphere National Bank and started, then slowed down, an ambitious campaign to organize Perpetual American Federal Savings and Loan Association workers.

The Hemisphere campaign gained the local some notoriety among law firms that help banks fight unionization drives. A lawyers' training manual distributed at a seminar hosted by the Washington firm of Arent Fox Kintner Plotkin & Kahn included a chapter entitled "Union Literature," which was filled entirely with Local 400 leaflets from the Hemisphere campaign. McNutt keeps a copy of the manual -- "Unionization and Equal Employment Problems in Banks and Other Financial Institutions" -- on the desk in his office.

The biggest prize among the awards and certificates of appreciation that adorn his office wall is a brass reproduction of the NLRB tally sheet of the nationally noted 1979 election through which Local 400 unionized 6,000 Woodward & Lothrop employes in the Washington area. The copy is so exact that it includes the NLRB's typographical error, in which an "R" was typed over an "O" to correct the unintended "Woodward and Lothoop." The campaign was won with considerable financial aid from the international union and volunteer help from 500 of Local 400's own shop stewards.

The local also uses its numbers and power in the field of politics, and is legendary in Prince George's County, where organized labor's endorsement can be a ticket to victory in Democratic races. "If you have Local 400 behind you, you have COPE the AFL-CIO's political arm behind you," said State Sen. Jack Garrity (D-Hyattsville), a beneficiary of Local 400's endorsement in his last race. The local, one of the only ones in the area with its own full-time lobbyist, marshaled its considerable membership to campaign for Rep. Steny Hoyer in his recent election, and furnished space in its headquarters for a Hoyer phone bank manned by the labor movement.

It was also one of the leaders in the move to deny Virginia Lt. Gov. Charles S. Robb the labor endorsement in his Democratic gubernatorial campaign. But to many Virginians, Local 400 is best known for its general counsel, former Virginia State Del. Ira Lechner of Arlington, who twice has failed to gain the party's nomination as lieutenant governor despite the local's strong support. Last week, it issued endorsements in the Virginia House of Delegates races in Prince William, Manassas Park and Manassas. The day before, Local 400 was named the most politically active local at the Maryland-D.C. AFL-CIO convention.

One of its current campaigns, which combines the local's political influence with labor organizing, is targeted at Maryland's state troopers, whose starting yearly pay is $13,570. Top pay for troopers is $17,827, according to a spokesman, compared to $20,150 for a full-time retail clerk with more than two years of experience.

The seed of that campaign was planted, McNutt said, when he was driving home from Ocean City one weekend and saw a billboard, sponsored by some state troopers' wives, with a picture of a Maryland state policeman and the message: "He risks his life for you for $13,500."

Soon afterward, he heard reports that the state police had approached the Teamsters. When a few officers finally called him, he said, he was ready to pounce. He has since met with officers in Frederick, Salisbury, Easton, Pikesville, Cumberland and Hagerstown. McNutt said he plans to enlist his members to pressure Maryland lawmakers to support a bill making it possible for Local 400 to hold a union election among the troopers.

The biggest election now scheduled is at Makro, a self-service wholesale house selling food and other items in Largo. A subsidiary of the Dutch conglomerate SHV Holdings nv, Makro is the first firm lured to Prince George's under County Executive Lawrence Hogan's international marketing campaign. The election is scheduled for Sept. 11, and a Makro vice president declined comment until after election day.

Local 400 organizers have been working 20-hour days as the election approaches. They head before dawn for the large Makro parking lot off I-95, and they are there at closing time when the work force of more than 200 heads home. The Local 400 organizers, like the retail clerks, will be working today; but they will not get the same holiday pay since they are salaried, rather than hourly, workers.

The high Labor Day wages of retail clerks inspired a special leaflet that was to be distributed today in the Makro campaign. The leaflets, being printed over the weekend, were to ask Makro clerks as they headed home after their Labor Day labors: "What did you make today? Clerks at Giant, Safeway and A&P made $27.88 an hour."