Thanks to a lone Republican's sit-in and an incipient bipartisan rebellion, the fight over nerve gas is not over yet.

Rep. Ed Bethune (R-Ark.), a pro-defense conservative, sat hour after hour for three days last week, waiting to block any unanimous consent request to bring the $200 billion defense authorization bill--of which the new nerve gas buildup is a $130-million item--to the floor.

Friends from his own party relieved him from time to time in his vigil. But he couldn't read or chat or do anything but watch the doors so he could spot anyone who tried to slip through a waiver of the usual three-day waiting period required of conference committee reports.

Bethune's patience paid off. Congress recessed without taking final action on the defense bill.

Pentagon contractors are indignant about the delay. But chemical warfare foes hope to use the interval to rally opposition to the administration's insistence on breaking the 14-year moratorium on the production of lethal nerve gas.

What makes the coming renewal of hostilities even more appealing to doves is the MX-missile component. Authorization for the president's most wanted weapon is also in the conference report. And one pro-MX member is threatening to change his vote when the MX appropriation comes up in September.

Rep. Clement J. Zablocki (D-Wis.), chairman of the House Foreign Affairs Committee, has warned that if nerve gas remains in the defense bill he will take it out on the MX, to which he has always given the nod. He and Bethune were co-sponsors of the amendment to turn off the gas that won in the House by 15 votes in July.

Vice President Bush, of course, cast the tie-breaking vote for nerve gas in the Senate. The fight against it there was led by Sen. David H. Pryor (D-Ark.), even though the arsenal is in his state; Sen. Dale Bumpers (D-Ark.) went the other way. So did two presidential candidates, Sen. John Glenn (D-Ohio) and Sen. Ernest F. Hollings (D-S.C.).

Most opponents of chemical warfare rail that it is obscene.

Bethune, 47, an ex-FBI agent, prefers to point out that it is insane.

His research, which has taken him to Geneva to talk with arms-control negotiators and to the Edgewood Arsenal to meet with Chemical Corps experts, has persuaded him that we are blowing a propaganda opportunity to show the contrast between us and the Soviets in our approach to loathsome, anti-civilian weapons--and a chance to save millions on "nightmare contraptions" that won't work.

The new "Bigeye" chemical bomb, which is part of the buildup, exploded twice on the pad--a fact carefully concealed from Congress while the administration poured on the coal for another "deterrent."

The only thing that may deter the administration, which has gone all out for nerve gas, is the damage political fumes from the gas can do to the MX.

Bethune is for the MX, but he will risk it to get nerve gas considered separately.

His most valuable ally is Zablocki, a short and anxious man not given to boat-rocking.

After the conference committee report, Zablocki raged around the House. He was furious with Rep. Les Aspin (D-Wis.) for not representing his interests better.

He fumed at the the high-handed action taken by Rep. Samuel S. Stratton (D-N.Y.) who, after conferring with Senate Armed Services Committee Chairman John G. Tower (R-Tex.), entered the meeting, took out a piece of paper and grandly told the group, "Here's our position on nerve gas."

Zablocki issued a statement announcing that he would take a step unheard of for him by voting against the whole defense bill.

He privately told fellow members that he would so something more: vote against the MX on the next round. "And I'll take 10 votes with me."

MX opponents, who have been looking for big-name converts, are overjoyed.

"It's the administration's fault for linking nerve gas with MX," said Bethune. "They insisted on forcing them into the same bill. They don't want to have a separate vote on nerve gas because they know they would lose in the House again, and the program would be killed."

It's a cruel dilemma for the administration. It hates to give up any weapon, and it loves nerve gas because the weapon gives it a chance to talk about the Soviets and yellow rain. But of all its lethal new toys, the MX is by far the favorite because it is the centerpiece of the administration's curious arms-control "package."

But if the coalition--and the wrath of Zablocki--holds in September, the administration may have to choose.