The Reagan administration, citing the dangers of chemical warfare and the superiority of Soviet nerve-gas stockpiles, began a major campaign yesterday to persuade Congress to end a 16-year moratorium on production of chemical weapons.

Deputy Assistant Defense Secretary Thomas J. Welch told the Senate Armed Services Committee that the stockpile of U.S. chemical weapons, last tested in 1969, is "no longer useful on the battlefield. It was designed for a different war and a different fight plan. We do not have a militarily usable retaliatory capability."

The administration is seeking $174 million in fiscal 1986 to produce new chemical weapons it says are necessary to deter a Soviet nerve-gas attack in Europe.

Similar pleas have been rejected in the past three years, and congressional opponents in both chambers are gearing up for what is expected to be one of the most bitter fights over a major weapons system in President Reagan's $313 billion defense bill.

The size of the U.S. stockpile is classified but generally believed to number between 35,000 and 45,000 tons -- less than one-twentieth of the Soviet arsenal, according to Pentagon estimates.

Defense experts say little of the U.S. arsenal is "usable" because the Army no longer stocks weapons designed years ago to deploy the gas.

Reagan, pledging his "vigorous efforts" to correct the "asymmetry" in superpower stockpiles, sent a letter Tuesday to the Armed Services Committee chairman, Sen. Barry Goldwater (R-Ariz.), in which he emphasized his commitment to "regaining a chemical warfare deterrent."

Welch, who heads the Pentagon's chemical weapons division, led a panel of military officials who underlined the dangers if deterrence failed.

Retired general Frederick J. Kroesen, former commander of U.S. Army forces in Europe, testified that the impact of chemical warfare on an unprotected civilian population would be "devastating." He said U.S. forces equipped with protective gloves, masks and uniforms would be able to survive, but their effectiveness would be greatly diminished by the heavy equipment.

Even if U.S. forces were able to retaliate, he said, the Soviets would be able to prevail because of their larger stockpiles.

"We do not have any retaliatory capability to subject the enemy to the same kind of degradation to his military capability as we would suffer," Kroesen told the committee.

To dramatize the problem, Air Force Capt. Mike Carlson, a fighter pilot, showed committee members what it takes to prepare for a nerve-gas attack. He donned three pairs of gloves, a heavy mask and long underwear and covered himself in a large plastic bag -- increasing the weight of his equipment by 20 percent.

"There's trouble in visibility, dexterity with the fingers and dehydration," he said.

Welch emphasized the safety advantages of the chemical weapons sought by the administration: the Bigeye gas bomb and 155mm artillery shells.

Although the gas leak last December in Bhopal, India, that killed more than 2,000 persons has nothing to do with chemical weapons, the tragedy has raised questions about the safety of U.S. nerve-gas stockpiles, Welch said.

Stored in large tanks in eight states, the gas is carefully guarded and monitored for leaks at an annual cost of $50 million.

The proposed weapons, known as binaries, are regarded as safer to handle, transport and deploy because they are made of two nontoxic chemicals that become lethal only when mixed during firing.

"We don't have safety problems," said Welch, speaking of the current stockpiles. "We have safety concerns."

"We had a great many inquiries after Bhopal. That gas is nowhere near the lethality of the gas in our depots," he said.