Two convoys of ships carrying Soviet arms from Cuba have been secretly unloaded in Marxist Nicaragua to help build a growing weapons cache there for use in the coming battle for neighboring El Salvado, a development that may force beleaguered President Carter to reconsider his courtship of the left in Central America.

This secret supply undercuts the administration's policy of aiding Nicaragua's Sandinista regime in hope of preventing its total embrace of Moscow and Havana. Similarly, the administration has adopted the leftist cause in El Saalvador, while giving a cold shoulder to anti-communist elements.

Thus, the revelation of Marxist Nicaragua's turning into a staging area for subverting El Salvador puts a hard Caribbean choice to Carter. Should he try to rally national support for himself on this issue; he would have to sacrifice his own police. Yet senators in close touch with the worsening Caribbean crisis, such as Democrat Richard Stone of Florida, may demand exactly that.

Officials here believe the arms sent to Nicaragua are earmarked for use by Marxist factions in the battle for El Salvador. That is the next intended victim of Soviet-backed insurgency in what used to be Uncle Sam's back yard.

Exactly what equipment was delivered is not yet known to U.S. intelligence officials, who described the unopened crates as containing "heavy eqiupment." What these sources do know is that arms already delivered to Nicaragua include Soviet tanks and long-range artillery pieces. Like the arms in the newly disclosed two conveys, all came from Cuba.

For Jimmy Carter, this continuing evidence of Soviet-Cuban intentions to dominate the Caribbean comes at a precarious political moment in an area of critical sensitivity. If detente is dead elsewhere, Carter's men have worked hard to insulate Central America from the Cold War.

There has been a pattern to Carter decisions in the beleaguered Caribbean since he courted Cuba's Fidel Castro by canceling a U.S. naval exercise at Guantanamo Bay in January 1977. Time and again, Jimmy Carter has either looked away or explained away each provocation: the sinister Nicaraguan mission to Moscow early this year; the appearance in Cuban waters last year of two Soviet submarines; the late 1978 discovery that Moscow was arming Cuba with late-model MiG23s potentially capable of nuclear delivery.

U.S. diplomats in Central America -- especially Nicaragua and El Salvador -- have operated on orders that amount to this: a warm embrace for the left, a cold should to the right. Those who did not go along were removed. In successfully pressing for aid to Nicaragua, the administration unsuccessfully tried to get it on an unconditional basis with no democratic procedures required.

Carter may decide that the new Soviet arms challenge should not be dismissed with wordy assurances that all is well in the Caribbean. With opposition to his renomination rising and his standing in the polls sinking to record lows, Carter may react sharply to this new challenge from the Moscow-Havana axis.

After Carter backed out of his demand last September that the Soviet brigade be withdrawn from Cuba, the Iran and Afghanistan crisis revived his political fortunes just in time for the primary season. Could Central America do the same now even if it means standing his present policy on its head?

But the case for action transcends election-year politics. "The Russians and the Cubans are testing, testing," one high-level official told us. If Carter allows this newest test to pass unchallenged, he added, no Caribbean country up to and including Mexico can fail to get the message. "It's up for grabs and they're doing the grabbing."

One Democratic senator who backs Carter's reelection will put this demand to Carter: make a complete disclosure to the American people; cut off the $65 million in U.S. aid now available to the Sandinista regime (and block an additional $70 million aid package now moving through Congress); prepare for action, including a naval blockade, if Castro again thumbs his nose at the White House.

Such actions might help Carter's fallen political fortunes. Far more important, they would finally show a glint of Carter steel at the U.S. back door that would match the genuine efforts he is making to block further Soviets encroachments in more distant parts.