This year's battle between the Pentagon and Congress over resuming the production of nerve gas was joined yesterday before a Senate subcommittee. Defense Department witnesses insisted the step was essential, and critics countered that it would be overkill.

President Reagan notified Congress Feb. 8 that he intended to end the 1969 ban that then-President Nixon imposed on producing deadly chemical weapons. Reagan said a dangerous gap had opened between U.S. and Soviet capabilities in the chemical warfare area.

Richard N. Perle, an assistant secretary of defense, elaborated on that argument before the Senate Armed Services Subcommittee on Strategic and Nuclear Forces.

"Our chemical modernization program is essential to the national interest and is fully supportive of arms control efforts," Perle said.

U.S. restraint in suspending production of chemical weapons has failed to inspire the Soviets to do likewise, Perle added. He argued that modernizing the U.S. arsenal of such silent killers as nerve gas offered the best chance of prodding the Soviets to negotiate a verifiable ban on the development and production of such weapons.

Largely because of one-sided U.S. restraint, "the Soviet Union today possesses a decisive military advantage because of the large asymmetry in chemical capabilities," said another secretary of defense at the witness table, Richard L. Wagner. This edge, he contended, amounts to an open invitation to the Soviets to resort to chemical weapons.

"Chemical warfare represents our greatest vulnerability," Army Maj. Gen. Niles J. Fulwyler said in urging approval of the $30 million to gear up for nerve-gas production as well as millions to provide protective clothing for U.S. troops.

Military witnesses agreed with the assertion of Sen. David H. Pryor (D-Ark.) that Congress had given the Defense Department practically everything it had requested in protective measures against chemical attack. Pryor is a leading Senate opponent to resumed production of deadly nerve gas.

Pryor stressed that there already was a mountain of deadly chemical weapons stored up. Even more important, he said, is the need to try harder to negotiate a ban on the development and production of chemical weapons with the Soviet Union.