Claiming financial hardship, Potomac Electric Power Co. yesterday filed for a 4.3 percent rate increase in the District of Columbia on top of a previous request that is still awaiting a decision by the Public Service Commission.

Yesterday's request was for a $15.5 million increase. The previous request, made 18 months ago, was for $45 million, or nearly 16 percent.

Taken together, the two requests would add $4.20 to an average monthly residential bill of $19.91, according to the company.

"It's pretty outrageous," said D.C. People's Counsel Brian Lederer of the new request. "The company decides how much they want out of the ratepayers and then they go get it."

Lederer, who represents consumers in utility cases, said Pepco is in good financial shape and is overbuilding generating capacity -- the main reason for its need for big rate increases.

Lederer argued in the original case that rates in the District should be rolled back 7 percent, or $20.7 million.

In filing for a second increase in rates yesterday, Pepco said that earning per share for the first month of 1979 dropped from 11 to 7 cents per share compared with the same period in 1978.

A news release quoted Pepco Chairman W. Reid Thompson as saying that the "principal reason for the earnings decline is the extraordinary delay" by the District commission in deciding the first case.

Commission special counsel Melvin J. Washington said the new case "will be treated in the normal course of business."

He said the first case hasn't been decided yet by the three-member commission because of its "size, bulk and issues... It's just a massive, over-whelming case."

Sources close to the commission said the reason the first case hasn't been decided is that Chairman Elizabeth Hayes Patterson last fall fired a lawyer who was doing the work of putting together an opinion for the commissioners to sign.

Patterson did this shortly after becoming chairman because she thought the lawyer shouldn't write the opinion after having represented the commission staff, the sources said. Technically, the staff was a party to the case.

Then Patterson borrowed three young lawyers from the Federal Energy Regulatory Commission, the sources said. These lawyers started all over again studying the voluminous record and preparing to write the opinion.