The supposedly secret U.S. aid to anti-Soviet insurgents in Afghanistan has won increasingly prominent political support in this country at a time when charges are being made that the aid is insufficient and ineffective.

In a change from their 1980 positions, the national platforms of both the Democratic and Republican parties recently endorsed U.S. support for the Afghan "freedom fighters."

The 1984 Democratic platform went so far as to endorse "material assistance" for the insurgents.

Such aid was secretly begun in the last months of the Carter administration and continued to be furnished in the Reagan administration, but without public acknowledgment.

The thin wall of deniability on U.S. assistance was breached in late July, when the House Appropriations Committee gave widely publicized approval to a supplemental grant of $50 million in aid to the rebels. Lawmakers who traveled to Pakistan, where most of the exile groups are based, apparently generated the additional assistance.

According to the Federation for American Afghan Action, a lobbying organization calling for more and better U.S. aid to the resistance, $325 million in CIA funds has been spent on this program since it began shortly after the December 1979 Soviet invasion. The Afghanistan program is the largest covert CIA operation, according to a congressional source.

Andrew L. Evia, executive director of the lobbying group and a former U.S. Army captain, charged that he found many U.S.-supplied arms to be of poor quality or in poor condition when he inspected them in Pakistan several times in 1980-82.

"Our Afghan friends, while appreciating the support, confirmed that the aid that has reached them was for the most part useless for their tasks," Eiva told the Republican Platform Committee in Dallas last month.

After a Washington Post query, Eiva produced lists of "arms received" and "weapons distributed." He said it had been supplied to him by one of the 10 Afghan resistance factions headquartered in Pakistan.

According to the documents, this faction, which Eiva declined to name, received 7,577 old-fashioned Lee-Enfield rifles, 2,940 Kalashnikov automatic rifles, 331 RPG7 rocket launchers, 114 RPG234 rocket launchers, 355 pistols, 108 heavy machine guns, 30 82mm mortars and 19 light machine guns in 1981, 1982 and the first seven months of 1983. Most of the weapons were of Soviet design or manufacture. The document indicated that about one-third of the automatic rifles and pistols, and a substantial number of rocket launchers, were purchased from Chinese sources.

Eiva and a number of other sources said that the CIA insists on supplying Soviet weaponry rather than U.S. arms in order to maintain "plausible deniability" that the arms to fight the Soviet military are being furnished by Washington.

The CIA declined comment on Eiva's charges. The agency is caught in a "Catch 22" situation: it cannot refute the claims of ineffectiveness without admitting participation in the resistance. A June 11 Time magazine article, which reportedly includes CIA-furnished information, depicted the covert arms supply as sophisticated and effective.

Mohammed Nabi Salehi, Washington representative of the Islamic Unity of Afghanistan Mujahadeen, which includes three of the 10 resistance groups, said, "What we have received has been so pathetically inadequate and ineffective, it is a condemnation to slow death."

He said his groups' greatest need is for more advanced air defense rockets than the Soviet-made SAM7 weapons already supplied. He said they are needed "to protect ourselves against armed helicopters and MiG fighters."

Doubts about the effectiveness of the rebels's arms do not seem to be shared by the Soviet Union, which reportedly has lost increasing numbers of helicopters, airplanes and tanks in recent fighting and which has stepped up pressure on Pakistan to cut off the arms flow.

A Soviet diplomatic note July 1 to Pakistan charged that country with "direct participation" in the supply of arms to the insurgents and said there has been a recent increase in the supply.

Pakistani sources said the note declared that, because of the presence of Soviet troops and civilian experts in Afghanistan, Moscow regarded the weapons as directed against the Soviet Union. "These actions cannot be left without consequences," the Soviet message reportedly said.

The Pakistani government, which officially denies that it is aiding or abetting the resistance, rejected the Soviet allegations.

The Soviet warning, along with an increase in cross-border air and artillery attacks on Pakistani territory, seems to have caused new worry in Pakistan. At the request of the government of President Mohammed Zia ul-Haq, the State Department on Aug. 23 issued a statement condemning recent cross-border attacks and renewing U.S. support for the independence and territorial integrity of Pakistan.

Similar statements of official support were issued in London, Paris, Tokyo and Ankara, according to State Department sources.

The growing U.S. political support for aid to the Afghan insurgents, along with the escalating apprehension of Pakistan and increasing impatience of the Soviets, may be elements in a forthcoming debate over a resolution that would put Congress on record in favor of "material aid, as the United States considers appropriate, to help the Afghan people fight effectively for their freedom."

The resolution is cosponsored by 69 senators and 161 representatives of both parties, including a large number of Democratic liberals. Sen. Paul E. Tsongas (D-Mass.), its original sponsor, said he will seek to bring the resolution to a vote during the current session of Congress.

Asked for comment about the Tsongas resolution, the State Department said last week that it "shares congressional concern for the welfare of the Afghan people and welcomes the interest which would be manifest in a joint congressional resolution." However, the department said, "we are concerned that it could be misinterpreted and may not accomplish its objective."

The department implied that a resolution of backing for the Afghan resistance could feed Soviet "propaganda efforts" maintaining that outside interference, fostered by the West, is the cause of the fighting in Afghanistan rather than the Soviet military occupation.