There is disturbing evidence that the Soviets, encouraged by Jimmy Carter's tepid leadership, may be preparing another Cuban missile crisis for the United States.

The appearance of Soviet missiles in Cuba in 1962 brought a nuclear showdown between Nikita Khrushchev and John F. Kennedy. Kennedy faced down Khrushchev, who never recovered from the loss of face. Now with Carter in the White House, the Kremlin czars apparently think they can win a new Cuban confrontation.

Last year, American intelligence experts, after painstakingly putting together bits and pieces of evidence, informed the White House that there was a Soviet combat brigade in Cuba.

President Carter, with his eye on public opinion polls showing that he was regarded as a weak, ineffectual leader, rushed off half-cocked and announced with chest-thumping bravado that the presence of 2,600 Russian troops in Cuba was "unacceptable."

The Kremlin disdainfully dismissed Carter's saber-rattling as empty rhetoric, and when the Soviet combat brigade suddenly became "acceptable," Soviet strategists were confirmed in their suspicion that Carter was no tiger, but a pussycat.

The president's humiliation over the Soviet brigade issue was lost sight of by the American public within a matter of weeks, swallowed up in the greater crisis over the hostage seizure in Iran.

Now, less than a year after the combat brigade fiasco, U.S. intelligence analysts have compiled evidence that the Soviets are secretly developing a nuclear capability in Cuba -- may, in fact, already have introduced nuclear weapons into their satellite outpost 90 miles from Florida.

The reason the American public has not heard of this ominous development is that the men in the intelligence agencies fear the disclosure would touch off another international crisis -- one that Carter is ill-equipped to handle.

There is, of course, the possibility that the evidence is misleading. But the accumulation of data from various sources points to a "worstcase" projection that the russians are indeed creating a nuclear arsenal in Cuba -- and worstcase scenarios are the only safe ones to assume when dealing with the Kremlin.

One of the most disturbing developments was reported by a high-level Cuban defector, who until recently was being trained by the Russians to command a SAM2 (surface-to-air-missile) site. He told his intelligence debriefers that the Soviets have modified their Cuban missiles by adding three booster rocket motors that increase their range threefold.

Intelligience sources told my associate Dale Van Atta the Cuban defector had been informed that the purpose of the SAM2 modification was to give it surface-to-surface capability. In other words, it can now be used for attack as well as defense. The defector reported that the modified SAM2s in Cuba can reach targets in Florida.

What alarmed the intelligence experts even more was the defector's report that he was told the new missiles' twin warheads were "very powerful" and required Soviet personnel to activate them. He was told the warheads are so powerful, in fact, that they were to be detonated only at maximum range.

The defector assumed from these hints that the missile warheads were nuclear. What he didn't know was that the Soviets had tested nuclear warheads of up to 25 kilotons on SAM2s as long ago as 1961, and that there are SAM2s deployed in the Soviet Union that are nuclear-armed.

Intelligience sources also noted that the Russians routinely describe their weapons as simply "immensely powerful" to disguise their nuclear reality from the troops who handle them. So the defector's reports add up to the distinct possibility that Soviet nuclear missiles are in position in Cuba at this very moment.

Another recent development adds to the overall picture. In 1978, President Carter expressed concern at the appearance of 20-odd Soviet MiG23s in Cuba. But intelligence experts determined they were for Cuban defense only.

What the American public was never told was that the Russians can rewire a MiG23 to carry nuclear weapons in approximately two days. Furthermore, the Soviets have constructed about 50 hardened shelters for the MiGs in Cuba -- and unusual precaution for a climate as mild as Cuba's. Analysts suggest that the strong shelters are intended to protect the MiG force from surveillance and/or attack by U.S. aircraft and missiles.

Perhaps the most alarming evidence of Soviet military preparations in Cuba is the construction that has been taking place at Matanzas, not far from Havanna. Initially, the activity around Matanzas was nearly dismissed as "suburban construction." But it now appears that what the Soviets are building there is indeed an underground bunker and/or silo for surface-to-air missiles. With the known nuclear capability of Soviet SAM2s, and the threefold increase in the missiles' range reported by the Cuban defector, this adds up to another Cuban missile crisis.