Contract negotiations between representatives of Woodward & Lothrop and a local retail union broke off yesterday, largely over the issue of union security.

Spokesmen for both camps acknowledged that negotiations between the department store chain and AFL-CIO Retail Store Employes Union Local 400 had opened smoothly two months ago but had broken off indefinitely.

The union has scheduled a meeting for Sunday to consider its next move in the dispute.

At least 15 union organizers were arrested yesterday in Woodward & Lothrop stores around metropolitan Washington on trespassing and other charges, according to Edwin K. Hoffman, chairman of the chain.

Hoffman accused the union representatives of "blasting into our stores," although Thomas McNutt, president of Local 400, said the company is "doing whatever they can think of" to "polarize" Woodies employes.

The major issue in the dispute appears to be union security, or the right of employes to join the union voluntarily.

'We were 85 percent of the way through," Hoffman said. "We can't satisfy them on that issue."

Hoffman said that Woodies' employes are transfered regularly among stores in Maryland, the District and Virginia. A right-to-work law in Virginia bars compulsary union membership.

According to McNutt, however, Woodies has "refused throughout the negotiations to address cost-of-living" and other issues vital to the union's position.

"They may want to force us into a strike, which if lengthy and unresolvable, would dissipate our strenght," McNutt said.

After a massive organizing effort, Local 400 won the biggest union organizing vote in the area's history in July, winning representation by a 4-to-1 margin.

Previously the company's workers had been represented by an independent union.That contract expired on June 30.