Some neighbors of the Capitol have begun to mobilize against what they view as a new threat that Congress will wipe out their row-house community to provide sites for moe multimillion-dollar House office buildings.

"We are something of an endangered species," Maurice Rosenblatt told a gethering of neighbors and lawmakers last week in his home at 421 New Jersey Ave. SE, four blocks south of the Capitol, where the expansion plans were discussed.

Elliott Carroll, executive assistant to the architect of the Capitol, said a proposal for at least one new building will be made this fall. He called opposition at this time "premature."

Two years ago, the neighborhood - whose homes largely date to the period after the Civil War - won a victory over a proposed building project.

At that time, the House appropriations Committee refused to spend $22.5 million to acquire property on New Jersey Avenue for a fourth building for House members' offices. It ordered the preparation of a master plan for future Capitol Hill development.

Each of nearly 100 visitors to Rosenblatt's home was handed copies of five potential development plans prepared by consultants to Capitol Architect George M. White as part of the master planning effort. All of the plans depicted office complexes sprawling across several blocks and extending southward almost to the Southeast Freeway.

Rosenblatt and others, including Rep. Robert L. F. Sikes (D-Fla.), who lives in the area, urged residents to contact individual congressmen and lobby against the proposals.

Sikes observed, only half in jest, that House leaders seem to feel the need for a fourth office building because they want to keep ahead of the Senate, which is now constructing its third office building.

After hearing Sikes, freshman Rep. David Cornwell (D-Ind.) said he is flatly opposed to expansion of congressional offices, "and so are my constitutents."

House offices are now located in a phalanx of three structures that lines Independence Avenue in the block just south of the Capitol. The buildings, all named for a former House speaker, are the Cannon (completed in 1908), the Longworth, (1933) and the Rayburn (1965).

"Citizens just normally assume the worst," Carroll told a reporter durign an interview on the forthcoming proposals. He said some of the current oppostion is hysterical.

Carroll said the plans now being circulated were made public at a community meeting March 5, and represent potential growth - which may never occur - over the next 50 years.

"I would like to dispel the idea that the architect (White) is planning the equivalent of eight new Rayburn buildings," Carroll said. "Actually, what is needed now is about 1.5 million square feet of space, avout the same usable space as a Rayburn."

The growth in future years would be slowed or speeded by decisions Congress itself will make in the future on how many more employees will be added to its payrolls.

A total of 21,000 persons now work on Capitol Hill, a figure that includes the work force at the Library of Congress and the Supreme Court as well as of Congress itself.

Although the membership of Congress has ramained virtually stable since 1910 (435 representatives, a few nonvoting delegates from territories and the District of Columbia and two senators per state), the size of congressional staffs has risen sharply.

At the start of the Roosevelt New Deal in 1933, a House member ordinarily had one secretary. By 1950, a typical office had about five employees. Today, each office is authorized 18, although some work in members' district office back home. Committee staffs have grown, too.

Congress itself has begun to look at the way it uses its office space. A unit called the House Commission on Information and Facilities, headed by Rep. Jack Brooks (D-Texas), recently recommended a reshuffling of offices and the location of some activities to locations not on Capitol Hill itself.

In addition to the informal gathering at Rosenblatt's home, community groups have begun to adopt formal positions generally critical of congressional expansion.

In an 11-point resolution, the Capitol Hill Restoration Society called for basic reforms in the congressional use of space. It said Congress should help preserve the Capitol Hill historic district, which includes the New Jersey Avenue area.

The Commentator, a newsletter published by the local advisory neighborhood commission, said the area "is in for trouble . . . no matter what (construction) alternatives are ultimately chosen."