RONALD REAGAN gambled in putting off for nearly 18 months his return to the strategic arms negotiating table. He figured to use the time to crank up a big arms-building program. What happened, however, was that large swaths of his public, American and European, became restless. As a result, though Mr. Reagan has been getting most of what he has sought so far in arms, he has had to mollify sentiment for a "freeze," sentiment that would hinder precisely the buildup he believes essential to erase a putative Soviet missile lead and to make Moscow take his negotiating proposals seriously.

This is the context in which Mr. Reagan has been striding eagerly toward the table he turned away from earlier. He reached it yesterday: START talks with the Soviets, aimed at the reduction rather than just the limitation of the nuclear weapons the two sides aim at each other, opened in Geneva.

The administration is not pleased to have the freeze movement on its back. It fears the Soviets will be encouraged to hang tough to see whether Mr. Reagan can keep his public with him into 1984. Still, the pressure on the president may not be altogether a bad thing--and not simply because it may compensate for the departure of Secretary of State Alexander Haig, the leading administration proponent of a moderate reading of President Reagan's strategic policy. The president brought much of his difficulty on himself; it took him about a year, for instance, to start adjusting his public utterances to political realities and to drop the impossible conditions he had set for resuming arms talks--that Washington first close the perceived Soviet missile lead and that Moscow first reform its international conduct. Even now, there is legitimate debate about the premise on which he is entering START.

The premise is that the Soviets have a "bulge," especially in the highly accurate land-based missiles theoretically suitable for a first strike. The administration hopes to induce Moscow to yield that advantage; in exchange, it would forgo some part of its new weapons plans. The Kremlin, however, takes another view. It acknowledges its lead in the branch of special administration concern but argues that Americans excel in branches of special Soviet concern and that overall the two sides stand at rough parity. Its negotiating stand reflects this no-bulge premise.

For all of the administration's anxieties about the peace and freeze movements, it does not appear, yet, to face anything like the opposition to SALT II that Ronald Reagan helped organize and rode to the White House. That means the president does have the opportunity to test his negotiating strategy. In urging the Soviets to look carefully at his proposals, he promised to look carefully at theirs. Therein lies a basis for hope that, no matter how far apart the two sides are as they start START, they can come closer as the talks go on. STYLE PLUS FINDS By Jura Koncius PHOTO(By Ray Lustig--The Washington Post) This wooden barnyard on wheels was designed with antique pull toys in mind. $13 each. Needlepoint Designers/Country Sampler, 1055 Thomas Jefferson St. NW.