Mikhail Gorbachev's hold on power in the Soviet Union seems to be getting increasingly shaky with each passing day.

So shaky, in fact, that the West had better start drawing up contingency plans for the time when the Kremlin is under new management - or at least a different style of management. The alternatives that need to be considered by such contingency plans include:- A new Soviet leader with generally the same outlook and objectives as Gorbachev but without the burden of internal unpopularity he has gradually accumulated since taking over in 1985.

- The attempted abandonment of the reforms of the past five years and a reversion to a strong-arm government, with or without Gorbachev.

The difficulty of Gorbachev's already slippery position became even more intense this week when the Soviet president addressed the Supreme Soviet legislature in response to its demands for a full report on his plans for political and economic change.

In that pivotal address, Gorbachev announced plans for yet another in a long series of bloodless purges involving the ouster of military and government officials resisting his efforts to replace communism with a watered-down form of capitalism.

In response, members of the Supreme Soviet refused to applaud and rose to ask questions - an act of defiance that would have been unthinkable a few years ago. Gorbachev left without answering.

But then all sorts of unthinkable developments are underway in the Soviet Union. Fourteen of the USSR's 15 constituent republics have either announced their independence or their intention to seek it. Even Soviet people who do not want to break away are increasingly unhappy with Gorbachev because of the serious disintegration of the economy. Food rationing will begin next month in Leningrad, the USSR's second largest city. Some smaller cities have lived with rationing for more than a year. In Moscow, items such as sugar are restricted and the city council is thinking of issuing coupon booklets. At the same time, Gorbachev is locked in a power struggle with Russian Federation President Boris Yeltsin over competing plans for economic reform.

The West can, of course, try to prop up Gorbachev by sending food shipments to the Soviets - but not without running the risk of undercutting his position with a clear demonstration of his dependence on outsiders.

For now, Gorbachev still holds the levers of power. But if Washington and other world capitals are wise, they will start opening lines of communication to other parts of the Soviet system, including leaders in the dissident republics.