THE ARMS TREATY now all but ready for summit signing is bringing President Reagan something less than the full-throated congratulations he might have expected for a successful negotiation. And it was in major ways a successful negotiation. In very heavy weather Mr. Reagan held the Alliance together and made the Soviets back down in their bald attempt at the nuclear intimidation of Europe. The agreement he got does not merely top off certain categories of weapons but eliminates them. It provides for unequal reductions in areas of Soviet numerical superiority in order to achieve a balanced result. It includes verification measures of unprecedented sweep and intrusiveness. It contributes to strategic stability by removing the American weapons that could hit Moscow so quickly that the Soviets, seeing or fearing their coming in a crisis, might fire off a big barrage.

Why then are so many people so grim and unhappy?

Some say it's because the agreement really is no big deal and may be more trouble than it's worth. It covers only 4 percent of Soviet warheads, whose targets can easily be covered by other warheads. Its verification, notwithstanding the 200 pages of fine print, is not foolproof. It raises Alliance-straining questions of whether Europe is losing the previous degree of certainty of American nuclear patronage and of whether NATO can henceforth hold up its end in conventional arms, an area in which the East has some important advantages. The problem is ''not so much INF,'' says Sen. James McClure (R-Idaho), ''but what's next.''

But come now. INF (intermediate-range nuclear forces) was never supposed to solve the West's every strategic and political dilemma. It was a response to an urgent Soviet-created problem that Ronald Reagan inherited. It turns out to be the political gateway to other questions, which have to be dealt with on their merits. The acknowledged limitations of INF are not so much crippling flaws as incentives to get on with other business. This is ignored by the company of naysayers that includes many of the original one-dimensional Reaganites, like Sen. McClure, and most of the Republican presidential hopefuls. With increasing boldness, they now patronize their ostensible chief as an apostate and as a lame duck seeking to revive a fading presidency by an arms-control fling with Mikhail Gorbachev. Democrats are more supportive but wish to put some of their own fingerprints on Mr. Reagan's diplomatic handiwork.

All this promises to complicate INF ratification and to tighten the pressure on Mr. Reagan as he tackles other questions with Mr. Gorbachev. But INF is an agreement of value, and it should not be lost in the political shuffle.