Metro opponents, armed with some interoffice memorandums from the U.S. Department of Transportation are attacking as defective a regional task force analysis of the Metro subway that resulted in a federal commitment to support the "goal of completing" the planned 100-mile system.

A group called Coalition for Metro Accountability (COMA), whose chairman Edward Kanwit waged a vigorous battle against expanding Metro during hearings last spring, obtained the documents under the federal Freedom of Information Act.

They key document, a July 24 memorandum from Chester Davenport, assistant transportation secretary for policy and international affairs, said the restudy of Metro has "a number of serious defects which prevent it from being an effective decision-making document."

On Aug. 17, Transportation Secretary Brock Adams declared the restudy completed and said that the Carter administration stood firmly behind "the goal of completing" the 100-mile system. He said that U.S. aid for the system would be dependent on up-front guarantees of millions of dollars from local and state governments and the resolution of several questions that still are under negotiation.

A Prince George's County group that is loosely allied with Kanwit's organization passed out a letter at a County Council work session Monday stating that no rationale could be found for Adams' decision in the voluminous files on Metro.

That group, called Citizens Organized for Unification and Preservation of our Neighborhoods, is headed by June Weber Miller, who also organized the successful drive in the recent election requiring a referendum before future county bonds for Metro could be sold.

Davenport's memorandum and other supporting papers charged among other things that Metro's potential ridership was dramatically overestimated, that fare assumptions were unrealistic, that operating costs were understated and that the restudy failed to break out costs and benefits of each uncompleted and unfunded segment of the subway.

Many of those points were addressed in Metro staff work after the final submission of the study, however.

Richard S. Page, administrator of the Urban Mass Transportation Administration and Adams'key lieutenant in the Metro study, wrote to area officials on Sept. 19 that the study did have some faults, but that "nonetheless . . . the study was through and useful to us as well as to local officials."

In an interview yesterday, Page said that "if we listened to every single analyst in this building we would never make any decisions . . . There is no such thing as a perfect alternatives analysis and there never will be."

Page said that local governments and their staffs worked "much harder than any federal bureaucrat" on the study, which lasted almost two years, and that "we decided . . . it was time for us to stop raising nit-picking questions."

The federal commitment was supported, Adams said, by the White House and the Office of Management and Budget. President Carter was briefed on the matter and approved the commitment, Adams said.