MOSCOW, DEC. 29 -- Perestroika, Mikhail Gorbachev's attempt to turn the Soviet Union from a totalitarian state into a law-governed democracy, has reached a dramatic climax. The upbeat first act has ended on an ominous note, with sharp reversals of fortune for many of the original characters. As the curtain falls for the intermission, new people have appeared on stage and the spectators are full of suspense.

As historians look back on 1990, they are likely to detect a fundamental shift in Soviet politics. The conservatives -- a motley assortment of Communist Party apparatchiks, army generals, nationalist Russian writers and managers of state-run enterprises -- have seized the political initiative for the first time since Gorbachev came to power in 1985. The reformist coalition of radical intellectuals, populist politicians and leaders of the non-Russian liberation movements is in danger of falling apart.

The year began with Gorbachev still pushing forward the boundaries of perestroika, persuading the Communist Party to give up its monopoly on political power. Opposition forces swept to power in six of the Soviet Union's 15 republics, including Russia itself. By August, Gorbachev was talking about the need to form a "center-left coalition" to push through radical economic reforms. (In Soviet political jargon, "left" means radical or progressive.)

The year ended with the reformers on the defensive and Gorbachev forming a tactical alliance with hard-liners in the army and party in the name of "restoring order." The president's rightward shift was underlined by the dramatic resignation of Foreign Minister Eduard Shevardnadze last week in protest against the onset of dictatorship. "Comrade democrats," Shevardnadze declared, in one of the most electrifying moments of the perestroika drama, "you have scattered. The reformers have gone to ground."

Paradoxically, it now turns out that the electoral triumph of the radicals marked the beginning of their political decline. As long as they were in opposition, the democrats were able to pin all the blame for the Soviet Union's economic problems on the Communist Party. As soon as they occupied their new offices in Moscow, Leningrad and other big cities, they also assumed responsibility for the deepening crisis, without the power to do very much about it.

"The democrats are panicking," said Ales Adamovich, a progressive film-maker and deputy in the full Soviet parliament. "They understand that the conservatives control the army, the KGB, the Communist Party, the entire state-run economy. Their only trump card is popular support, but public opinion can be extremely fickle."

When populist politician Boris Yeltsin was elected president of Russia last May, he appealed for a two-year mandate of public trust to put the economy back on its feet. At first, many ordinary Russians seemed disposed to grant him his request. But their confidence has been shaken by the collapse of his promised transition to a market economy before the 500-day countdown even began.

"There haven't been any real reforms in Russia and, what's more, there aren't going to be," said Boris Fyodorov, one of the authors of the 500-day program, after resigning as Yeltsin's finance minister this week. "We are deceiving people by promising reforms and not carrying them out. With the policy that we are implementing, we are not going to restore the health of our finances."

The most startling statistic of 1990 was the first officially acknowledged decline in the Soviet Union's national income in more than four decades. Industrial production shrank by 3 percent. The amount of money printed by the state bank was twice the planned level. If present trends continue, Soviet economists predict a catastrophic recession in 1991, with real living standards declining by as much as one-third.

If electoral success was a curse for the radicals, electoral defeat proved a blessing in disguise for the conservatives. Initially, many party apparatchiks were psychologically unprepared for perestroika and glasnost, Gorbachev's policy of openness. Reared in an atmosphere of backstage intrigue, they suddenly found themselves obliged to defend themselves in public for the first time in their careers. Loss of office galvanized the conservatives into action, forcing them to close ranks for the coming counteroffensive.

Under the influence of last year's revolutions in Eastern Europe, the radicals quit the ruling Communist Party in droves. In doing so, they have made it easier for the conservatives to take over an institution that still controls the principal levers of power in the Soviet Union. The Moscow party organization, previously regarded as a liberal stronghold, has now publicly aligned itself with the hard-line Russian Communist Party.

A similar mistake was made by Poland's Solidarity movement in 1981, prior to the imposition of martial law by Gen. Wojciech Jaruzelski. Flush with popular support, Solidarity strategists argued that there was little point in getting involved in the internal intrigues of a discredited Communist Party. What they forgot was that the party still possessed repressive powers that could be used against them in a showdown.

The Soviet Communist Party has certainly not succeeded in regaining any of its lost prestige among the population. Its rating in opinion polls has slumped to single figures. But by purging its old-guard leaders and making some token concessions to democracy, it has regained a sense of its own political legitimacy. It no longer feels paralyzed.

A significant nuance has crept into the rhetoric of perestroika that helps explain why Shevardnadze felt it was time to go. Kremlin officials are no longer insisting that they will rely on "exclusively political means" to solve the Soviet Union's problems. Last week, KGB chief Vladimir Kryuchkov came up with a new formula: Political methods are certainly "preferable," but the leadership reserves the right to use force as a last resort.

Up until the fall, there appeared to be a good chance that the Soviet Union would succeed in making a rapid break with the Stalinist command economy. The tentative agreement of all 15 republics on the 500-day program, which envisaged a forced march toward the market through a massive sale of state assets, was a moment of hope that is unlikely to be repeated soon. But the hope foundered on Yeltsin's populism -- his insistence that economic reform could be achieved without sacrifices from ordinary people -- and on Gorbachev's instinctive political caution.

"You must remember that Gorbachev is a product of the Soviet political system and is unable to make a decisive break with it. If he had been a true radical, he would not have survived and climbed to the top of the ladder. That's why he is now parting company with his reformist allies and turning to others for advice," said Alexei Emelyanov, a radical economist.

The prospect of a dramatic shift of economic resources to the private sector alarmed the leaders of the Soviet military-industrial complex, the vast supply system for the Soviet Union's 4 million-strong armed forces. Defense industry chiefs persuaded Gorbachev that it was simply too risky to dismantle the most efficient and productive sector of the Soviet economy. The generals weighed in with warnings about the collapsing morale of the army and the threat of ethnic breakup.

A year ago, it seemed possible that the anti-Communist revolution that shook Eastern Europe in 1989 could be repeated in the Soviet Union, the world's first socialist country. This now seems unlikely, at least in the near future. In theory, economic chaos could result in massive street disorders. But, given the present alignment of political forces, any popular uprising would probably be sharply suppressed, paving the way for a hard-line regime.

The key question now is whether the conservatives will succeed in consummating their political success inside the Kremlin with a victory in the country. Gorbachev may have taken a significant step in their direction, but the suspicion is that he is playing for time. The father of perestroika is probably calculating that, after a necessary interval for order and discipline to be restored, he will continue with his grand reformist strategy.

In the short term, a return to some kind of authoritarian rule seems quite possible, even probable. The optimists are predicting a relatively mild presidential regime headed by Gorbachev himself in the name of saving perestroika. The pessimists are talking about "National Bolshevism," based on a mixture of offended Russian nationalism and vengeful communism.

"The chances for successful democratization, which were never very great, have clearly dwindled," wrote political commentator Lilya Shevtsova in a gloomy review of 1990 for the progressive weekly Novaya Vremya. "It's doubtful that we will be able to create in the near future a liberal democracy with division of powers, rule of law and the guarantee of individual rights and freedoms. The most elementary conditions do not exist."

Alexander Lyubimov, the host of the popular TV news show "Vzglyad" and a deputy in the Russian parliament, was slightly more upbeat. "A return to authoritarian rule is quite likely in the Slavic heartland and the Central Asian republics. But I can't see it happening in the Baltics and the Transcaucasus where the independence movements have come to power. It would involve too much bloodshed."