Two national unions are seeking to replace a long-established local labor organization at Potomac Electric Power Co., which represents about 3,500 workers at the Washington-based electric utility.

In an election being supervised by the national Labor Relations Board, ballots were mailed from Baltimore this week after a spirited three-way campign that started when enough of Pepco's workers sought new representation last month.

A three-year contract for the Pepco work force expires at rhe end of May,and no negotiations have been held pending the outcome of the unions battle.

Since the 1930s, the utility's white collar and blue collar workers have been represented by the independent Electric Utility Employees Union, which has had no formal ties to national labor organizations

In the current contest, the EUEU is being challenged by two AFL-CIO units:

The huge International Brotherhood of Electrical Owrkers (IBEW), with more than 1 million members across the country and representative for workers at a majority of the private utilities that are organized, including Virginia Electric & Power Co.IBEW members also work in construction and related businesses.

The Utility Workers Union of America (UWUA), with some 55,000 members-95 percent of them utility employes. The UWUA lost a pepco organization attempt three years ago and vowed at that time to return.

Officers of the three unions all said yesterday they expect to win, but the possibilty exists that none will reveive the required majority of those voting. In that event, the NLRB is expected to conduct a runoff election between the two largest vote-getters.

W.E. Collins, secretary-treasure of EUEU, called the three-way union fight "a fiasco" that is only delaying bargaining for a new contract. The two national unions, Collins asserted, "dont't operate with the best interests of the's a money game for them."

Collins' union won by a comfortable margin when challenged three years ago, but a large number of Pepco's workers didn't vote at that time. Union representatives said they expect a higher turnout this time.

IBEW organization director Michael Lucas said yesterday that his union is seeking jurisdiction only at Pepco but also at Baltimore Gas & Electric Co., where there is no union. Pepco and BG& & E are the only major utility firms in the Pennsylvania-New Jersey-Maryland power pool where workers are not under an AFL-CIO unit. BG & E has had no union, and UBEW has started to survey workers there, Lucas said.

John Moriarity, national representative for the UWUA, noted that his organization has been working with a group of Pepco employes since before the 1976 election drive. "They are dissatisfied, having rejected a company contract and then been forced to accept it," he added.

A key point in the national union organizing drives has been Pepco's wage and fringe benefit package-which is said to trail thoise of other utilities in the regional power pool In the latest year for which figures are available, 1976, average Pepco workers earned $15,781 in wages and $2,232 in fringes compared with Public Service Electric & Gas of New Jersey,where IBEW workers earned $17,885 in wages and $4,093 in fringes.

Pepco spokesman David Boyce said his firm has staked out a "hands-off" policy on the union fight, "which is not really any of ourbussiness...we will negotiate in good faith with whoever it is." Spokesmen for the unions said the company had interfered with the representation election.