CUBA'S ALREADY STAGGERING economy absorbed a hard, perhaps lethal, blow the other day. And oddly enough, the event failed to make the front pages or the evening television news shows.

The news was buried on the business pages of a few alert papers. Briefly, Russia's trade minister announced a halt to oil shipments to Cuba because the island didn't deliver enough sugar to Moscow.There could be no worse development for longtime dictator Fidel Castro. The oil-for-sugar swap is vital if Cuba's economy is going to keep limping along.

In Castro's palmier days, the Soviet Union subsidized Cuba's petroleum needs in return for a communist base and showcase in the Western Hemisphere.

But the Soviet Union collapsed and Russia is nearly broke. Sensibly, it put its trade links with Cuba on a business basis.

Last December the two nations agreed to barter 2.5 million tons of Russian oil for 1 million tons of Cuban sugar.

But Cuba has been plagued by poor crops. It absorbed 1.5 million tons of Russian oil this year but delivered only 500,000 tons of sugar. On Nov. 1, the shortfall caused the Russian trade official, Oleg Davydov, to say enough is enough - no more oil.

Davydov said the 1 million tons of oil, formerly earmarked for Cuba, would be sold on the world market, earning about $120 million for Russia.

Even with Russian oil deliveries, Cuba descended into crisis this year. As much as 80 percent of its factories are closed because of shortages of fuel, raw materials or spare parts.

If Castro cannot replace the Russian supplies, the crisis will grow worse: Power plants, industries, transport and farms will grind to a halt. Nearby Mexico and Venezuela can provide the needed oil, but whether they will is unclear since Cuba may be unable to pay.

The centrally planned communist economy that Castro has boasted about for 35 years is in a vicious cycle.

Sugar is the principal export, but scarce fertilizer, fuel and spare parts depress output. Also, he pays sugar workers with near-worthless pesos, so they refuse to work hard.

Cuba's sugar production was a healthy 7 million tons in the 1991-92 crop year. It fell to 4.2 million tons in 1992-93 and 4 million tons in 1993-94. The current harvest, which begins this month, may not top 3 million tons.

In fact, some experts think Cuba will be unable to deliver the 500,000 tons it has promised to China. The result might be a lack of Chinese consumer goods as well as Russian oil.

Last summer, shortages of food and other basic goods touched off rioting in downtown Havana and a flood of rafters toward the United States. Castro stopped the flow of refugees as part of an immigration deal with Washington.

He also has stepped up the repression of dissidents.

As living conditions fall, as they are bound to without Russian oil, more widespread anti-regime riots are likely. If they erupt, Cuban army officers face a cruel choice: Do they fire on their own people to save Fidel and Raul Castro and the officers' own privileged place in society?

Whatever the answer, Castroism looks doomed. It has given Cuba failed central planning, Eastern European-type factories that cannot compete and a losing system that tries to exchange cheap sugar for expensive oil and technology.

Castro needs - pardon the expression - a sugar daddy to replace Moscow. Can Washington be that vacuous?