Instead of launching their expected bitter-end struggle against ratification of the INF treaty, Senate conservatives were so surprised by inspection procedures agreed to in Geneva that they may move to a fallback strategy: wait for next year's START accord.

That is bowing to reality. The unexpected improvements in verification procedures make it impossible to defeat INF, which destroys medium-range missiles. So, conservative senators may retreat gracefully, some of them actually voting for ratification, and wait for START, the treaty to cut intercontinental weapons by half.

INF undercuts the NATO alliance and may diminish European security. But START impinges on the very existence of the United States as a sovereign power.

What's more, President Reagan's yearning for a long-range missile treaty is no longer pie in the sky. U.S. and Soviet negotiators who finished drafting INF this week have also moved with unanticipated speed to reach partial agreement on START. Thus, in the words of one hard-liner, INF becomes ''only the hors d'oeuvre before START.''

Instead of advertising weakness with a futile attempt to prevent INF ratification, its foes are considering using the debate to establish a tough new set of verification principles for the START negotiations. These will be modeled on lines of around-the-clock, on-site inspection for INF agreed to in Geneva.

But since the strategic arms treaty has national security implications that dwarf INF, the conservatives want START inspection to be squarely based on Moscow's unexpected Tuesday agreement -- but far more intrusive.

This has yielded a surprising list of Republican senators who have taken a tough line on arms control but now sit on the INF fence and may end up voting yes: James McClure, chairman of the Senate Republican Steering Committee; Malcolm Wallop, who worries that ratification failure would gravely compromise conservative leaders in Europe; Pete Wilson and Daniel Quayle, two defense-oriented newcomers who operate skillfully out of the limelight.

Without McClure taking an activist role in trying to bring down INF, any Senate floor campaign to defeat or hobble it would be doomed. Thus, it is critically important that McClure tells friends he might wind up supporting the treaty.

One possible reason: he is one of seven national cochairmen for Vice President George Bush's Republican presidential campaign. Bush is the only candidate who has come out for the new agreement (committing himself even before it was written).

Politics aside, although McClure has major reservations about INF, he was pleasantly surprised by the inspection system to be placed on a key Soviet nuclear assembly plant. He sees it as a low-grade sample of what the United States must insist on to prevent life-or-death Soviet violations of START.

If this trend holds, opposition to INF ratification may be limited to fewer than a dozen Republicans, headed by Sen. Jesse Helms, with one or two Democrats possibly joining them. Even if the trend does not hold, the stage has been so artfully set by Secretary of State George Shultz that conservatives who still want to defeat the treaty find themselves badly handicapped.

Reagan administration arms controllers have made sure there is no chance the president can meet the Dec. 1 deadline for reporting about Soviet cheating on existing nuclear treaties. At least 20 conservative Republicans wrote the president Sept. 21 asking that the Dec. 1 report be sent to Congress well before the summit and the INF signing.

Instead, the new compliance report, certain to provide ammunition against ratification, was stymied at the Arms Control and Disarmament Agency. Accordingly, continuing failure of the United States to get Mikhail Gorbachev to end Soviet violations cannot play a key role in the INF treaty.

Similarly, the five-year statutory review to Congress on operations of the Anti-Ballistic Missile Treaty has been smothered. The Republican right had formally requested it by Oct. 3, correctly viewing it, too, as a prime lever to break open chinks in the new agreement.

Even with the on-site inspection agreement, the administration remains unable to tell Senate treaty foes the whereabouts of an estimated 36 SS-20 missiles -- the main Soviet weapon to be destroyed under terms of the treaty. That will be pointed out in Senate debate by hard-line foes of the INF pact, offering amendments and reservations galore. But the battle may be over before it starts. So, it's wait for next year and START.