Washington — The political debate over nerve gas, which for months has had more than a whiff of controversy, is becoming more pungent. There are allegations that the Pentagon has been withholding data on test failures of new chemical weapons sought by the Reagan administration; there are moves in Congress to sharply cut appropriations for such weapons; and new doubts are being raised about the alleged use of ''yellow rain'' by the Soviet Union.
It has been 14 years since the United States built any new chemical weapons. The Pentagon says its arsenal is aging and unreliable and warns that the Soviet Union has a bristling chemical-warfare capability that can't be ignored.
Moscow or its allies have used such weapons in Afghanistan, Laos, and Cambodia, they and would not hesitate to use them in any East-West military confrontation, US officials charge. In order to deter such a threat, it is argued, US forces must be able to protect themselves and retaliate.
Congress recently authorized the first installment on a $6 billion to $7 billion administration five-year plan to build new bombs, artillery shells, and other chemical equipment. Charges of Soviet ''barbarity'' in the wake of the shooting down of the South Korean airliner helped the administration win in this case.
But doubts persist about proceeding with new chemical weapons here, aside from what many see as a moral question over breaking a self-imposed moratorium on weapons that all agree are particularly repulsive.
A new book, ''The Yellow Rainmakers'' by Grant Evans, calls into question charges that communist clients are using Soviet-made chemical weapons against tribesmen in Laos. Based on interviews with refugees, Australian sociologist Evans concludes that doubts about the US charges - at least in Laos - are well-founded.
Here in Washington, Rep. Ed Bethune (R) of Arkansas this week charged that the Pentagon ''intentionally kept from Congress'' research and development problems with the ''Bigeye'' chemical bomb, that ''members were duped into voting for production of the Bigeye.''
The Pentagon says this bomb is safer because it is ''binary.'' That is, relatively harmless chemicals are not mixed together into deadly combination until after the bomb is dropped (or artillery shell is fired). But there have been testing problems acknowledged to be of ''serious concern.'' The Pentagon says there were some ''leaks'' in testing; Congressman Bethune (near whose Arkansas district the weapons are to be built) says he has evidence that there was an ''explosion.''
''If Congress had known of the failures . . . there is no doubt in my mind that more than a year ago the Bigeye bomb would have been seen for what it is - a mechanical nightmare,'' he wrote in a letter to the General Accounting Office (GAO) this week.
Bethune wants the GAO, the investigative arm of Congress, to find out the truth about chemical-weapons testing, whether Congress was kept in the dark, and whether the matter should ''be referred to the Justice Department or the Inspector General for possible criminal, civil, or military justice code sanctions.''
Although Congress has already authorized the 1984 funding of chemical weapons , the actual appropriations have yet to be approved. The House Defense Appropriations Subcommittee recently cut by nearly half the amount sought for nerve gas weapons production. If Bethune's charges are true, it may be some time before the political air is cleared over chemical weapons.