TODAY the Soviet Union is in the final stages of making a historic choice of a structure to bind its constituent republics into one nation. After three drafts, a new treaty of union of the republics is virtually ready for signing, though not without the resolution of some remaining disputes.What kind of system have the Soviets chosen? A close look at the new document reveals a curious - and at times, contradictory - blend of federation and confederation. It invests the republics with sovereignty, but it also maintains a strong central government with considerable powers. The mixture reflects the complex history that led up to this moment and the intense political struggle that has surrounded the drafting process. The new treaty replaces the 1922 treaty, the product of the Bolshevik Revolution of 1917 and the civil war that followed. That document enshrined Bolshevik leader Vladimir Lenin's rejection of the imperialism of Russia in favor of national self-determination. In theory, the republics are organized along national lines, with each nation voluntarily joining the Communist union. In reality, particularly under Joseph Stalin's dictatorship, the republican divisions were convenient tools to divide and rule. And the Soviet Union was a ruthlessly centralized dictatorship. Now the revived nationalist movements that have taken power in many republics demand that power be given to them, in effect fulfilling the original Leninist idea in a loose confederation. The succession of three drafts - the first last November, the second in March, and the latest in mid-June - have steadily moved from centralized federation to a more confederal arrangment. The republics, led by the most powerful, such as the Russian Federation, the Ukraine, and Kazakhstan, have chipped away at central authority. But they have also settled for an ambiguity, where agreement was absent, that augers for troubles to come. There are three key issues that reveal the fuzzy deal embodied in the treaty - the division of jurisdiction between union and republics; taxation and property; the structure of the federal government. The treaty defines areas of union jurisdiction and of joint jurisdiction that are far from clearly separated. (See box.) The federal government, for example, has the power to carry out foreign policy and defense. But the republics are given authority to define those policies. The union approves and executes the federal budget as well as monetary policy, but the republics "control" this process. The draft gives republics control over property, but says the union government receives the property it needs to carry out its powers. This could suggest, for example, that all defense industries are union property, an interpretation that some republican leaders already dispute. The property issue is closely linked to taxes. The draft creates a two-channel tax system, with both republican and federal taxes. In practice, taxes are mostly paid by state-owned enterprises in the form of paying most of their profits to the state. So ownership of the enterprise amounts to the right to tax. Both the Russian and the Ukrainian governments have already stated they oppose any federal tax and view all enterprises on their soil as their property. They argue for contributing funds to the federal government for those union programs to which they agree. Already, retorts Soviet Premier Valentin Pavlov, the refusal of these two governments to give agreed sums to the federal budget has caused a huge deficit. Soviet leader Mikhail Gorbachev and some republics also argue that dividing enterprises amounts to maintaining a state-controlled economy rather than moving to a market system. Kazakhstan President Nursultan Nazarbayev calls this demand for republican jurisdiction "populism." "What is jurisdiction?" he asks. "The enterprises should follow the laws of a particular territory. That is all." Perhaps most troubling the draft provides no executive mechanism for implementing joint powers. A previous draft created a Federation Council, which grouped republican leaders with the union president in a decisionmaking body. Now the only integrative structure is a two-chamber legislature: a Soviet of Republics and a Soviet of the Union. The Soviet of the Republics is formed along the same lines as the current Soviet of Nationalities, with a fixed, equal number of representatives for each republic and for each of the smaller autonomous republics and regions, national units within republics. That house, like the United States Senate, passes treaties and votes on Cabinet and high court nominations. The other house, whose members are elected like US representatives, can pass laws over the objection of the republican body but only with a two -thirds majority. Otherwise, all joint decisionmaking will take place in special committees or by separate agreement, none of which is actually spelled out in the current draft treaty. It is not difficult to anticipate that the Soviet leadership will find itself in a short time confronted with the need to significantly alter this historic document.