When President Reagan and Soviet leader Mikhail Gorbachev reached agreement last December on a treaty to ban intermediate-range nuclear missiles, the pact was almost universally cheered. But contrary to everyone's expectations, it has run into trouble in the U.S. Senate.

To be fair, the problems are not due to stubborn senators playing politics; many of the difficulties are being caused by the Russians. Given the Kremlin's long history of being recalcitrant, such snags are hardly unexpected.The INF treaty was hailed as "historic" at the time because it accomplished two things that had never been achieved by the superpowers: (1) It eliminated an entire class of nuclear weapons, and (2) it provided for on-site inspections to verify that no one was cheating. The latter provision is the heart of the treaty.

Unfortunately, the Soviets appear to be backing away when it comes to on-site inspections. At least they are doing their best to make the verification as narrow and limited as possible.

Soviet officials, like those scheduled to be stationed at Magna, and their American counterparts in Russia, are supposed make sure that intermediate-range missiles are not produced at missile plants.

But the Kremlin is now raising questions over details of access by American inspection teams and what those teams will be able to view.

Debate on the treaty had been scheduled for this week, but Republicans and Democrats joined hands to correctly shelve any talk about ratification until the inspection problems are resolved.

This could mean that Reagan will go to Moscow at the end of May without the ratified INF treaty in hand. While this may be a disappointment to the president, it would be equally frustrating to Gorbachev, who must be planning the Reagan visit as a boost for his own position. But security takes precedence over public relations.

A delay is not necessarily fatal; the treaty must be carefully worked out and accepted by both sides. The greater danger would be rushing to approve the INF treaty while leaving it full of loopholes.

Fortunately, there appears little risk of that. The decision to postpone debate has bipartisan support, and the Reagan administration seems to agree. Clearly, the treaty must insist on full and complete on-site inspections.

Once again, the INF treaty illustrates that signing an agreement with Moscow is not a cause for unrestrained optimism. What follows after is far more important, and far tougher to work out.