In addition to an undozing watchfulness against Soviet expansionism, real or imaged, Sen. John Tower (R-Texas), the chairman of the Armed Services Committee, also keeps sentry on his self-created Hypocrisy Watch. Five months ago, he wrote a patronizing letter to his colleagues in which he criticized "some senators" who want cuts in the military budget but who "argue just as strongly that such reductions should not be made in programs located in their states . . . In my 22 years in the Senate, I do not recall any senator ever volunteering such a budget reduction in his state."

A reply came from Sen. David Pryor (D-Ark.). With a wry touch that contrasted well with Tower's tone, Pryor said: "Fully recognizing that I will be ending the 22-year" record, "I want to recommend that the ($130 million chemical weapons) program in Pine Bluff, Ark., be cut from the 1984 defense budget."

The Tower-Pryor exchange, though five months past, is topical because in a few days Congress will decide whether it is time to escalate chemical warfare. The House has voted against resuming the production of nerve gas. The Senate--after a 49-49 vote that was broken by Vice President George Bush's vote in favor of production--has approved. A conference committee, to meet on the $200 billion 1984 weapons authorization bill, will settle the issue.

The exotic weaponry favored by the Reagan administration is binary nerve gas, a mixture that is stored in two compartments in artillery shells. The chemicals, on explosion, bring on gruesome death by disrupting nerve impulses to muscles and suffocation through destruction of the respiratory system. The repulsiveness of this type of killing is one of the reasons the United States has had a 14-year freeze on producing nerve gas.

David Pryor, whose love of Arkansas is matched by his skepticism about the Pentagon's reasoning that more nerve gas is needed to deter the Soviet Union's war lust, has been the most persuasive voice in Congress for continuing the ban. He has three main arguments, all worthy:

* The current stockpile is enormous: by one account, 9,000 square miles can be covered by the United States' nerve gas. Enough doses are available to kill every person on Earth. In Europe, where the gagging and dying is most likely to occur in an East-West war, the storage of poisonous gases has been rejected by NATO governments.

* Nerve gas won't kill soldiers, who have masks and protective equipment to wear, but civilians who don't. Scientific American magazine reported in 1980 that "civilian casualties on the order of a million could result from battlefield chemical warfare in Europe."

* To let the military dig in with the Pine Bluff production plant is to give it $130 million today and a blank check tomorrow. Weapons production doesn't start small and stay small. It begins big and gets bigger. Pryor estimates that in only a few years, between $4 billion and $6 billion could be given over to chemical warfare.

Public awareness of the Reagan administration's avidness for nerve gas has been low. The MX missile and B1 bomber dominate the war-preparation debate. The nuclear freeze movement commands attention because the holocaust of nukes is graphically imaginable as The End, while armies spraying gas at each other is seen as trifling war games on remote battlefields. For some, it's more than that: in an era of unwinnable nuclear war, direct combat between soldiers trying to kill each other face-to-face and mask-to-mask is an old-fashioned virtue that we ought to cherish.

In David Pryor, the country has a trustful and fair-minded guide in the defense of continuing the ban on nerve-gas production. He acknowledges that the citizens of Pine Bluff are split in their views. He has been explaining that if potential jobs are the issue--about 200 would be provided by the plant--then much more could be done against unemployment by using the money for his state's nonmilitary industries.

In the debate on the Senate floor, when it was Pryor against Tower, the standard oversimplifications against Pentagon critics had no meaning. Pryor is not known as a dove. He has voted for increases in military programs. He wants only to maintain the ban on further production.

Pryor, a reserved, thoughtful man who avoids polemics, has not stressed the moral arguments against nerve gas. But they underlie his case. When used in World War I, poisonous gas was the original unthinkable weapon. It remains that, except now it is one of many.