The Soviet Union is surreptitiously arming Cuba with Mig23 aircraft of the type now deployed in Europe for nuclear attack against NATO, a development casting a long shadow on President Carter's hope for Senate ratification next year of a strategic arms limitation treaty.

In a top-secret memorandum informing President Carter of this on Oct. 23, Defense Secretary Harold Brown specifically raised the question whether supplying Cuba with the high-performance Mig23 violates the "understandings" between President Kennedy and the Kremlin that ended the Cuban missile crisis in October 1962. Those understandings forced Moscow to remove ballistic missiles and other nuclear-armed weapons from Cuba, including the Badger bomber.

According to U.S. intelligence findings on which Brown based his memorandum to the president, the Cuba-based Mig23 has the configuration of the nuclear-attack aircraft that makes up the Warsaw Pact's most advanced nuclear-delivery tactical strike force.

There was no suggestion in Brown's memorandum to Carter that the Cubabased Mig23 - believed now to number some 12 to 15 aircraft - have actually been fitted or "wired," in the technical phrase, for carrying a nuclear payload. What can be said is that the Mig23 now in Cuba appears to be the same Model D or F long observed in the Warsaw's Pact's nuclear-delivery training exercises an attack, not an air-defense, aircraft.

Top Carter administration officials, not concealing their shock at discovering the Mig23 with its one-way 1,200-mile range based 90 miles off the Florida coast, are now hotly debating what to do and say about it. As Brown warned Carter in his memo, the totally unexpected Soviet move presents the president with a problem of "high political sensitivity."

To make the fact public poses this unpleasant choice: Demand that Moscow immediately remove all Mig23s, as the U.S. successfully demanded of the Soviet missiles and Badger bombers in 1962, when the strategic balance was overwhelmingly in Washington's favor, or try to explain it away as a mere upgrading or modernization of Cuba's air force, whose Mig21s are now largely in the service of Cuba's African adventures.

If Carter chose the first option, odds are prohibitive that Moscow would tell him to go jump in the lake, diplomatically or not, and probably make it stick. That would bring an instant crisis in detente and put the new SALT agreement in jeopardy.

But to accept the presence of the Mig23 in Cuba, knowing its apparent capability for being fitted for nuclear-delivery, would violate the doctrine laid down by John F. Kennedy during the Cuban missile crisis and by the Nixon-Kissinger strategists in 1970.

In that year, Richard Nixon and his then-national security adviser Henry Kissinger delivered what amounted to an ultimatum against Soviet-Cuban construction of submarine pens at Cienfuegos capable of servicing nuclear-missile submarines. That ultimatum was based squarely on the Kennedy doctrine any nuclear-capable weapon - whether it was actually fitted with nuclear armaments or not - was ipso facto intolerable in Fidel Castro's Cuba. The submarine pens were not built.

It is doubtful that even a signed Cuban "pledge" that its new Mig23s would never be equipped for nuclear delivery would do much to solve Carter's political problem in the Senate when the new SALT agreement comes up for debate next year. With the southern part of the United States exposed to attack within the Mig23's nonstop, 1,200-mile range, the SALT debate might well turn into an anti-Soviet free-for-all.

Indeed, the shrewdest analysts here are hard put to explain the surreptitious deployment of the Mig23 in terms other than Soviet muscle-flexing, mixed with deterrence. It would seem to provide Moscow with a powerful option to be brandished when and if necessary as a counter to American policy anywhere in the world considered hostile by the Soviet Union.

U.S. defense officials say that the Mig23 model now in Cuba has never before been delivered to a Soviet ally. After the Arab-Israeli war in October 1973, Syria received the air-defense version of the Mig23 - essentially a fighter plane not equipped for tactical strikes against ground targets. The difference is similar to that between the U.S. F4 and the F5E. The F4 has been denied to Taiwan because it could attack the Chinese mainland; the F5E, a plane designed purely for defense, is being supplied instead.

That the Soviets have chosen this critical moment in off-again, on-again detente to threaten the United States in its own back yard is a vicious irony for Jimmy Carter. Unless he can conjure up a justifiable explanation for the Mig23s where none appears today, his hopes for SALT II may turn to dust.