Facing a deadline of midnight tonight to reach a new contract, negotiators for Washington and Baltimore supermarkets and their 20,000 employes talked long into last night without reaching agreement.

There was no threat of a strike, however, and federal mediator Donald Brodsky said chances of a settlement were good "if the past M.O. holds up."

The local supermarkets and their employes traditionally negotiate until the eleventh hour. "In the food industry, they don't shoot until they see the whites of our eyes coming up the hill," a union spokesman said.

At stakes are wages and fringe benefits for cashiers, clerks, meat cutters and other workers in most major supermarkets.

The stores negotiate jointly under the umbrella of the Food Employers Labor Relations Association, which includes Safeway, Giant, A&P, Grand Union, Food Fair/Pantry Pride and Memco, but not Acme which dropped out of the group.

They are negotiating with Retail Clerks locals 400 in Washington and 692 in Baltimore and Amalgamated Meat Cutters locals 593 in Washington and 117 in Baltimore.

The 13,000 clerks now earn $6.69 an hour after two years experience and the members of the smaller meat cutters union earn somewhat more, a spokesman said.