MOSCOW, NOV. 17 -- At the heart of the political drama now unfolding in the Kremlin are two radically different concept of the nature of the Soviet state and the ultimate purpose of President Mikhail Gorbachev's perestroika reform movement.

Apart from Gorbachev himself, the chief actors are Russian President Boris Yeltsin and Soviet Prime Minister Nikolai Ryzhkov. Both politically and physically, they are a study in contrasts. Ryzhkov, suave and soft-spoken, stands for the preservation of the unitary Soviet state. Yeltsin, bulky and brash, is demanding the dismantling of the Soviet Union in its present form and its replacement by a much looser confederation of republics.

The struggle between these two contrasting visions of how a country with 280 million people and more than 100 distinct ethnic groups should be organized has been underway for the past two years, partly in the open, partly in secret. It came to a head this week after it finally became clear that Gorbachev's repeated attempts to bridge the differences between the two principal protagonists -- each representing powerful political forces -- had led to a paralysis of power.

In their speeches to the Soviet legislature, Ryzhkov and Yeltsin agreed that the country is on the verge of chaos. But while Ryzhkov blamed the crisis on "separatists" and "chauvinists" in the Soviet Union's 15 constituent republics, Yeltsin pinned the blame on the central government and the "totalitarian" political structure created by the ruling Communist Party.

"There are many political currents in this country that would like to destroy the union," insisted Ryzhkov, 61, who became premier in 1985 after a successful career as an industrial manager. "The federal government has been an obstacle to their aspirations. That is why it is now being subjected to such harsh criticism."

Ryzhkov made clear that he regards himself as the defender of state traditions that date back "many centuries" before the Communists came to power in 1917. He also aligned himself with Gorbachev's conservative critics in the party in arguing that perestroika has lost much of its original content. He said that "destructive forces" were using the slogans of perestroika, which had aimed at the renewal of socialism, to change the Soviet Union's constitutional order.

The coalition of forces supporting Ryzhkov includes the bulk of the party and government bureaucracy, the senior ranks of the armed forces, and what Soviet commentators describe as "the military-industrial complex." Aggressive lobbying by the directors of many Soviet defense factories who fear production would be disrupted by too rapid a shift to a market economy helped stave off demands for Ryzhkov's resignation earlier this fall.

For Yeltsin, who announced his resignation from the Communist Party at the landmark 28th Party Congress last June, there is no point in trying to fix a system that does not work. The populist Russian leader has called for the dismantling of Communist cells in all state institutions and a rapid transition to a Western-style democracy with a mixed economy. Yeltsin's formula for keeping the Soviet Union together is to allow its constituent parts to go their own ways in the hope that economic self-interest will then oblige the separate components to join together voluntarily in a new economic union, somewhat similar to the European Community.

In his speech Friday, Yeltsin depicted the national paralysis of power as a result of a "crisis of the totalitarian system that exists to this day."

"The old center is trying to save itself at any price," he said, "not giving the republics any possibility of conducting their affairs more independently. The center in its old form has clearly become destabilizing."

The most important source of Yeltsin's authority is the support he enjoys from Soviet citizens impressed by his battles with the bureaucracy. Public opinion polls constantly rate him the most popular politician in the country. Other elements in the pro-Yeltsin alliance include the new democratic parties opposed to Communist domination and the national liberation movements in the outlying Soviet republics.

In a way, Yeltsin and Ryzhkov represent two facets of Gorbachev's own split personality. This may be one reason why he finds it so difficult to come down firmly on one side or the other. The initiator of perestroika seems perpetually torn between the popular aspirations he helped release and political debts to the bureaucratic apparatus from which he sprang.

But Gorbachev's hesitancy in resolving his dilemma is more than just a matter of psychology or power politics. It also involves great issues of state. As Soviet president, he bears ultimate responsibility for the security and territorial integrity of a world superpower, and he is acutely aware that a wrong step could spell disaster.

The policies advocated by Yeltsin and Ryzhkov both have obvious flaws. Yeltsin's approach -- he has called for every Soviet republic or region to have as "much sovereignty as it wants" -- could lead to political disintegration. Ryzhkov's insistence on preserving the prerogatives of the central government could easily become an excuse for maintaining the old command-administrative system.

On paper, the emergency state of the nation debate in the legislature seems to have produced a victory for Yeltsin. The federal government stucture, together with the post of prime minister, will be dismantled in favor of a presidential cabinet. The individual republics will be represented on a strengthened Federation Council with an executive arm to be known as the Inter-Republican Committee.

In practice, however, the struggle between the advocates of strong central government and the republics' sovereignty is far from over, and the competing political forces represented by Yeltsin and Ryzkhov are preparing for new skirmishes. Ryzhkov himself is an obvious candidate for the new post of vice president, but the battle for Gorbachev's soul goes on.