WASHINGTON (AP) _ In this city with a lobbyist for almost every cause, Afghanistan's rebel tribesmen have found champions on Capitol Hill for their five-year battle to oust Soviet invaders.

''We're fighting a war here,'' declares Mary Spencer who left her job as a medical assistant to crusade for the Afghans.

A former Green Beret, a woman paratrooper, a conservative ''free enterprise'' lobbyist and the son of an adviser to Indian independence leader Mohandas K. Gandhi also buttonhole lawmakers on behalf of the mountain nation's turbaned warriors.

Strong in their beliefs, at times blunter than the typical lobbyist, the self-proclaimed spokesmen for the Afghans when not blasting the Soviets at times swap insults with each other. A slander suit is in the courts.

''They keep calling us KGB agents,'' says Matt Eurlkar, executive director of the American-Afghan Education Fund. ''Do I sound like a KGB agent to you?''

The mood at times smacks of international intrique. The Indian-born Eurlkar envisions privately financed pilots, similar to the Flying Tigers of World War II, for clandestine arms flights to the rebels.

Miss Spencer of the Committee for a Free Afghanistan frets that Soviet disinformation agents lurk on Capitol Hill. Indeed, Arnaud de Borchgrave, author of ''The Spike,'' a novel about Soviet disinformation, is on on the committee's advisory council.

Senate staffers credit the Federation for American-Afghan Action and its executive director, former Green Beret Andrew Iva, for helping to drive through, over State Department opposition, a 1984 resolution sponsored by former Sen. Paul Tsongas, D-Mass. It called for active aid to the rebels.

The committee and federation have stimulated Senate interest by bringing rebels here to tell their story, the staffers say.

Rep. Henry J. Hyde, R-Ill., a House Foreign Affairs Committee member, says the groups ''keep alive an issue that otherwise might fall through the cracks.''

The Committee for a Free Afghanistan has helped to bring rebels here for medical treatment and raised funds to send doctors to the war zone. Its offices, provided by the conservative Heritage Foundation, generate lapel buttons, brochures and posters. One, headlined ''Disarmament Soviet Style,'' shows an Afghan boy with no arms.

This week, the group's executive director, Karen McKay, a former 82nd Airborne Division officer, was in Pakistan to meet with Afghan rebels, Miss Spencer said.

A chief target of the federation is the Central Intelligence Agency, responsible for funneling covert U.S. aid to the rebels, estimated at $250 million a year.

''They have bungled the job,'' declares federation Chairman Neal Blair, a partner of ''free enterprise'' newsletter magnate Howard Ruff.

''Either through malfeasance or misfeasance they have allowed the Pakistani government to take a lot of it, and a lot of it has been stolen on the black market,'' he declares, echoing a complaint of the rebels themselves.

Thus, a chief federation goal is to put weapons directly into the hands of the fighters, bypassing the Pakistani government and assorted Afghan politicians congregated on the Afghanistan-Pakistan border.

Meanwhile, members of a growing community of Afghan refugees clustered around the lobbying groups here say the United States must plug the leaky aid pipeline and provide better weapons or their fellow countrymen in the hills, under attack from Soviet jets and helicopters, will lose.

''Our people aren't made out of rocks,'' refugee Waleed Wardak, 21, tells a visitor to the Committee for a Free Afghanistan office. ''They're just ordinary human beings.''