The Senate will vote soon on a bill to expand the boundaries of the Capitol grounds in a way that would outlaw the vendors who now sell food and souvenirs on the Mall. The measure also could lead to an expansion of "permit only" parking for congressional employees.

The senate Rules and Administration Committee approved the measure last week, and Chairman Howard W. Cannon (D-Nev.) reported it to the floor with a request for action. It is now pending.

William M. Cochrane, committee staff director, said the bill's main purpose is to improve security by enlarging the area routinely patroled by the Capitol police force.

Although adding parking spaces is not the purpose of the measure, Cochrane acknowledged that the expansion of the Capitol grounds would make it legally possible to eliminate several blocks of parking for the general public and reserve the space for Capitol Hill workers.

The Senate on Monday defeated an attempt by Sen. Gary Hart (D-Colo.) to eliminate free parking in the 8,494 spaces on Capitol Hill and to start charging users.

Capitol Police Chief James M. Powell said the expansion measure, as written, would evict dozens of vendors who got permission from wagons and stands parked along Maryland and Pennsylvania Avenues where they cut into the Mall at the foot of Capitol Hill.

The vendors won that privilege last year after they voiced angry, public protests against being forced off other parts of the Mall by the National Park Service. There are about 3,000 licensed vendors in the city, and as many as 100 are said to use the new vending areas. About 30 were there yesterday.

Powell said federal law prohibits all private vending on the Capitol grounds, and the pending measure makes no exception.

In general, the proposal would extend the Capitol grounds - and the Capitol police jurisdiction - to cover the entire width of some streets at the outer edges of the existing 181-acre reservation. On several of those streets, the congressional jurisdiction now ends at the edge of the streets closest to the Capitol.

The boundaries generally are Union Station on the north, C and D streets SE-SW on the south, 2d Street Se-NE on the east and 1st Street NW-SW on the west.However, two triangular areas would be extended farther west, including the full widths of Maryland and Pennsylvania Avenues as far as 3d Street, embracing the areas used by vendors.

Cochrane, the Senate committee official, bristled at a reporter's question about the possible expansion of permit parking, saying "certainly that is not the motive." He conceded that the proposal would make such expansion legally possible.

With a single exception, there is no public parking on any street that is officially designated as part of the Capitol grounds. The exception is a one-block section of Massachusetts Avenue NE adjacent to Union Station, where about 40 metered spaces are available to the public.

At least 100 metered spaces and at least an equal number of unmetered spaces on the new boundary streets would become vulnerable to a possible expansion of congressional parking.

The meters are in the 100 block of D Street NE, along 2d Street NW from Massachusetts Avenue to F Street, on North Capitol Street from D Street NE-NW to Massachusetts Avenue on New Jersey Avenue from Louisiana Avenue to D Street NW, in the 200 block of Constitution Avenue NW in front of the Labor Department and on Canal Street SW adjoining the congressional Botanic Garden.