Warning that an issue before the District of Columbia Public Service Commission has "ramifications of national importance," President Carter's top consumer adviser stepped into a local utility regulation case for the first time.

The D.C. case could establish a national pattern for sharing part of the cost of nuclear power plant accidents, says the U.S. Office of Consumer Affairs, headed by White House consumer aide Esther Peterson.

The agency filed a formal petition to intervene in the D.C. Public Service Commission's deliberations on a request filed by Potomac Electric Power Co.

Pepco has asked permission to give what amount to special discounts on electricity it sells to Metropolitan Edison, the owners of the Three Mile Island nuclear power plant.

Since a radiation leak closed the Three Mile Island plant almost a year ago, Metropolitan Edison has been forced to buy large amounts of electricity; Pepco is one of its major suppliers.

Utility regulations require Pepco to share any profits it makes on such sales with its local customers, who pay -- through their monthly bills -- for building Pepco's power plants.

As a result, Pepco's customers will save about $2 on their electric bills over the next year, and its Maryland consumers will pay about 90 cents a month less for power for the next few months.

Part of those savings will be lost if Pepco is allowed to give a special discount to Metropolitan Edison. But Met Ed's customers will save money because the full cost of the purchased power is passed on to them.

"Consumers beyond the boundaries of the District of Columbia will be directly and indirectly affected by the outcome" of the D.C. decision on Pepco's request, the Office of Consumer Affairs noted in a petition to intervene in the case.

Office of Consumer Affairs attorney Harold T. Judd said the agency has made no decision about whether to support or oppose the discount to Metropolitan Edison.

The D.C. case is the first of several such requests filed with state regulators by companies selling power to Met Ed.

If the states agree to the discount, it could set a pattern for aiding utilities in the event of other nuclear accidents.

The state action eventually will have to be approved by the Federal Energy Regulatory Commission, but the utility companies have taken their case to local regulators rather than going directly to the federal agency.

Acknowledging that its District of Columbia customers would lose about $2 million in benefits if its request is approved, Pepco contends its discount plan has advantages for its customers. To get the discount, Metropolitan Edison would have to pay Pepco every week for power it buys; that would protect Pepco if Met Ed goes broke, Pepco says.