While Defense Secretary Frank Carlucci and National Security Adviser Colin Powell were refusing to give up on President Reagan's Strategic Defense Initiative, Mikhail Gorbachev was polishing a big surprise for the summit: an East-West pact against the horrors of chemical warfare.

Failure by Gorbachev and Reagan to agree at the current summit on SDI would derail detailed agreement on START, a landmark treaty cutting in half intercontinental missiles. Since Gorbachev wants a propaganda coup here transcending the INF treaty, he then could unveil his plan to ban chemical warfare -- likely to be rejected by Reagan as unverifiable. As at Reykjavik, the Soviet leader would confront his unprepared U.S. adversaries with a proposal requiring hard decisions here.

The chemical warfare gambit preserves momentum for Gorbachev's peace offensive, so appealing to Americans and Western Europeans. It also reiterates world-class Soviet resourcefulness in psychological warfare for Western hearts and minds.

The administration's split on SDI reached a climax late last week. On Thursday, Secretary of State George Shultz traveled to the Pentagon to talk the Joint Chiefs of Staff into agreeing to trade SDI for START. That, said Shultz, would be not only in the long-range interests of the United States but in the short-term interests of the military. Sitting with the chiefs in the ''Tank'' (the supersecure inner sanctum), Shultz explained how pleasant life for the brass would be on Capitol Hill with no SDI to defend.

Whether that appeal to self-interest worked, Shultz probably made some progress. But the next day, when the National Security Planning Group met in the White House Situation Room, all progress was unmade. The key reason was Carlucci, who said ''there is no meaningful distinction {on the matter of SDI} between me and Cap Weinberger.''

Strongly joining Carlucci was Powell, his successor as national security adviser. They blunted Shultz's protracted and steadily accelerating effort to wean the president from SDI. Although the secretary never quits and the president often changes, the decision on the eve of the summit was to hang tough against swapping SDI for START. Gorbachev has been ready for such a temporary roadblock to START. Preparations for a chemical warfare ban surfaced behind the scenes only last week at U.N.-sponsored disarmament negotiations in Geneva, when Soviet Ambassador Yuri Nazarkin, the top Soviet chemical warfare negotiator, suddenly left without notice. American diplomats learned he unexpectedly was being brought to Washington for the summit.

The second signal came in a question to a British delegate from another member of the Soviet delegation in Geneva. Igor Scherbach, reputed to be the KGB's top agent on the delegation, informally asked how Britain would react to a Soviet proposal for a chemical warfare ''pact to pact'' ban between the Warsaw Pact and NATO.

It would eliminate chemical warfare in a European ''free zone.'' If it surfaced as a ''happy surprise'' (in the sarcastic phrase of a U.S. policy-maker) at the Washington summit, Soviet propagandists could trumpet it to the embarrassment of the United States. Gorbachev's Reykjavik triumph, when he proposed elimination of nuclear weapons, would be repeated.

It surely would put the Reagan administration on the defensive, and it could not be ignored. The United States in perfunctory manner proposed a chemical warfare treaty nearly four years ago, though neither the Pentagon nor the State Department ever liked it. In a report two weeks ago, even de'tente-minded Democrats on the House Intelligence Committee warned that ''a mandatory challenge inspection regime'' -- the right to inspect any time anywhere -- would only ''marginally'' improve confidence against Soviet cheating.

Reagan advisers are split over the effectiveness of verification and whether intrusive, day-and-night monitoring could safeguard against cheating. Highly placed military officers believe the ban would be inherently unverifiable, no matter how intrusive the on-site inspection.

But the Soviets recently opened a quiet back channel between Soviet Foreign Minister Eduard Shevardnadze and Shultz for exploring the chemical warfare treaty. That cuts out the Pentagon, Central Intelligence Agency and other normal parties to national security policy-making. Over the back channel, Shultz has been told that Gorbachev is prepared to make ''an enormous concession'' to win American consent for a ''framework'' treaty this week.

If that ''enormous concession'' comes, the president, as at Reykjavik, would have the problem of saying no. Chemical and biological warfare is second only to nuclear weapons as mankind's greatest killer threat. Yet the president must consider the difficulty of verifying elimination of the massive Soviet stock, so menacing that Reagan finally overcame congressional opposition to production of binary bombs and shells for chemical agents.

That congressional switch helps explain Gorbachev's moves at the Geneva disarmament talks and the back channel to Shultz. The Kremlin chess master seems always one step ahead of his American counterparts. While they fought to switch their president on SDI, Gorbachev was preparing his daring new move in the game of global psychological warfare.