Federal mediators met yesterday with executives of Raleigh Stores Corp. and the union representing more than 500 employes in an effort to avert a threatened strike at the 12-store clothing chain.

"Raleigh's offer is totally inadequate, and unless we have an improvement, we will strike," Thomas McNutt, president of Local 400 of the United Food and Commercial Workers union, said during a late afternoon break in the negotiations.

Raleigh's officials were not available for comment yesterday, but Bernard Janicki, the firm's executive vice president, said in a recent interview there was "a good chance to avoid a strike if we use common sense and really sit down and try" to resolve the stalemate.

Sales commissions are the key issue in the dispute, according to McNutt, who said Raleigh has paid commissions equal to 8 percent of sales since 1981. He said the company is resisting efforts to increase the commission to 8 1/8 percent and instead is offering a one-time raise of $250 for a three-year contract, which he termed "ridiculous."

Janicki said the firm was offering "much more" than the lump sum because the proposed contract also would include improvements in pension and health benefits. He declined to elaborate, but McNutt said those improvements have come at relatively little cost to the company. Raleigh's sales are roughly $70 million yearly but profits of the privately held chain are not disclosed.

The dispute pits the Washington area's largest local union against the largest independent clothing chain, whose workers have belonged to the union for more than 40 of the company's 74 years. UFCW last struck Raleigh's in 1980 for 27 days in a bitter dispute that disrupted what had previously been a harmonious relationship.

UFCW members voted overwhelmingly last week to authorize its leaders to call a strike if the company's offer did not change, McNutt said. He said yesterday he would recommend a strike, which could begin as early as next week, "unless we have some movement" in the talks supervised by the Federal Mediation and Conciliation Service.

Charles Hale, who has worked for Raleigh's for 34 years and is a member of the union negotiating committee, said that "Raleigh's is a good firm and there is no animosity anywhere. But when you're talking about money and fringe benefits, management naturally wants to keep costs down, and we want to earn more to have a better life. That's the American way."

Salespeople earn an average of $32,000 in men's clothing and $20,000 in women's in a job that requires sales skill, long hours and hard work, McNutt said. Top salespeople earn up to $60,000, he said. Cashiers and nonsales employes earn about $5 hourly and have been offered a 5 percent raise, he said.