Local 1900 of the International Brotherhood of Electrical Workers will strike Potomac Electric and Power Co. "if progress is not made in negotiations" with the investor-owned utility, union leaders said yesterday.

This is the first time during the current contract dispute that Local 1900 leaders actually have indicated their willingness to call a strike.

A walkout by the local's 3,500 members will begin "when our union negotiating committee determines that no further progress in negotiations can be made, and the timing is right," the local's leaders said yesterday in a bargaining report.

Talks between the company and union broke off at 6:30 p.m. Monday after federal mediators decided the two sides were too far apart on economic and related issues to continue talking.

A representative of the Federal Mediation and Conciliation Service, which has conducted the negotiations for the past three weeks, said yesterday that the agency is "staying in touch" with the negotiators to be on top of any developments indicating a mutual desire to resume bargaining.

"They weren't making any progress on any settlement of any kind," the FMCS representative said. "Both sides have an agenda of things they feel they really have to have. But neither side has put forward a proposal that has met acceptance by the other."

Pepco officials last week reported making "some progress" in the talks with the union that began Feb. 25. But company officials implied yesterday that bargaining became mired in union politics.

Local 1900, which has represented Pepco workers since 1979, was scheduled to hold elections on June 1. That would have meant holding elections one day after the union's old three-year contract expired May 31. A contract unacceptable to the membership conceivably could have weakened the position of any candidate who was involved in the bargaining.

Pepco and the union failed to reach agreement May 31. Under a special clause, the contract stayed in effect until last Thursday at 6 p.m., after which the union had the right to call a strike.

Meanwhile, the union had postponed its elections date until today, independent sources said. But a ranking Local 1900 official said yesterday that the election has been postponed again. "You're close, but you get no cigar," the union official told a reporter who mentioned today as a possible election date. "The vote will be held soon," he said.

However, the union official vehemently denied speculation that Local 1900 politics was delaying talks. "We're not playing politics at all. All of the candidates have pulled behind our position in the negotiations," he said. Asked to explain the delay, the union official added: "We just want to do it right. We want a good contract."

Another independent source who is familiar with the negotiations said yesterday that, intended or not, the union improved its bargaining position by invoking the contract termination clause last week.

"The company is incurring all kinds of extra costs because it has been double-manning, keeping supervisors on round-the-clock duty" since the clause took effect, the source said.

A Pepco management memo written yesterday, and later obtained by The Washingon Post, said company supervisors "have been asked to work long hours" and that "many of you supervisors are remaining on company property during your non-working hours." The memo by Edward F. Mitchell, Pepco's vice president for operations, said the round-the-clock double staffing by 500 supervisors will continue until the strike threat disappears.

"We are joined together in a plan preparing us to take over operation of all company facilities in the event of a failure in negotiations and a walkout call by the union," Mitchell said. A Pepco spokesman said the company had no exact figures on what it was spending to implement parts of its emergency plan.