THE Soviet Union and its once-powerful central institutions are on the verge of disappearing. In their place a "commonwealth" of states is emerging, with an alliance of the three Slavic republics of Russia, the Ukraine, and Byelorussia at its core.The leaders of those states met for two days over the weekend in a dacha outside of the Byelorussian city of Brest to work out a formula for their future ties. Today the Slavic leaders will present what amounts to a fait accompli to Soviet President Mikhail Gorbachev at a meeting in the Kremlin, also to be attended by the president of Kazakhstan. More than half of Kazakhstan is inhabited by people of Slavic origin. These developments were precipitated by the 90 percent majority vote for independence Ukrainians cast in a referendum Dec. 1. That vote effectively ended the last hopes for a renewed political union among the 12 republics of the former Soviet Union. Nevertheless, Mr. Gorbachev renewed appeals for union last week, railing against "isolationists and separatists." He predicted war and economic chaos if those appeals are not met. Gorbachev has been echoed by aides such as Foreign Minister Eduard Shevardnadze, as well as by St. Petersburg Mayor Anatoli Sobchak. They warn of an impending coup by military-led hard-liners, feeding off social tensions as the hard winter sets in. The coup rumors were further fed by the sudden dismissal over the weekend of Soviet Chief of the General Staff Vladimir Lobov, a move prompted, informed analysts say, by his increasingly open opposition to military reform. While such ominous visions are not easily dismissed, many observers see such talk as the last attempts of a dying center to preserve its role. Some say the new commonwealth is likely to exclude any future for Gorbachev. Last week, the daily Izvestia accused the president of living in a "world of illusions" in resisting the new political realities. "Everyone knows in their hearts that this union is already doomed," Ukrainian President Leonid Kravchuk said on arrival in the Byelorussian capital of Minsk. "The republics have refused to voluntarily delegate to the center the powers which it has demanded of them," Russian President Boris Yeltsin told the Byelorussian parliament on Saturday. "Today we see the failure of the idea of a half-federation, half-confederation which would bind each state implicitly under a system of dual power." In a clear warning to Gorbachev, Mr. Yeltsin added that "the main thing is not to demand the impossible from each other at this point. If we do otherwise, any treaty, however correct, may turn into just a piece of paper." In the weekend talks, Yeltsin aimed to find common ground with the Ukraine, the second most wealthy and populous Soviet republic without whose participation, Yeltsin has said, any form of union is impossible. From a variety of preliminary reports before conclusion of their talks yesterday, the new formula will throw out any form of political union. In its place will be a set of economic agreements under the broad umbrella of the treaty to form an economic community signed by 10 of the 12 republics (but still not ratified). It deals with common policies on prices, taxation, customs, debt, banking, and currencies. "I can imagine a military-political agreement being added to the economic agreement, covering the Army, [and] strategic weapons," says Ivan Silayev, the head of the Inter-state Economic Committee. After several days of talks among senior republican officials, he announced agreement on a few key issues, including joint responsibility for the Soviet Union's foreign debt, management of pension and other social welfare funds, and ceilings on prices in inter-republican trade for some key basic goods such as f ood and energy products. The first key test of economic cooperation is the Russian plan to free most prices from state control beginning Dec. 16. The price liberalization is a cornerstone of Russia's radical reform plans. But republics have urged the move be delayed, arguing it will trigger massive inflation and that too few bank notes will be available to meet the anticipated price rises. While Yeltsin has shown some flexibility on the timing of price reforms, he also made it clear that Russia will not back off from its plans. "The practice of mending holes has outlived its usefulness," he said in his speech in Minsk. He argued against the idea that any republic "can fence itself off from these economic troubles." "The crisis spares no one, and the sooner we go over to joint actions, the less painful the reform will be," he said. The need to reform military ties among the republics was a factor behind the removal of General Lobov, who wrote a long article in the Army daily Red Star on Nov. 28 insisting on centralized armed forces and attacking "national egoism in the sphere of defense." His ouster was "first of all a decision made by Yeltsin, and for Gorbachev it was easy to agree," says Sergei Blagovolin, president of the independent Institute of National Security and Strategic Studies. The key military problem is joint control over the long- and short-range nuclear-armed missiles that are stationed in the three Slavic republics and Kazakhstan. Russian Deputy Defense Minister Alexander Tsalko said that in talks with Ukrainian officials in Moscow last week, the Ukraine said it wanted control of the weapons based on its territory. "For them it is possible insurance against a threat from Russia," says Mr. Blagovolin. "It is nonsense. Russia is not in a position to threaten anybody except itself."