IT has been convenient over the past five years for the West to think of the Soviet Union, led by Mikhail Gorbachev, as a single entity. But such "old thinking" no longer applies. Power has shifted, irreversibly it seems, to the Soviet republics.Next Sunday's independence referendum in the Ukraine throws the viability of the union further in doubt. The referendum seems certain to pass, and it appears current Ukrainian leader Leonid Kravchuk will be elected president. If the nuclear-armed Ukraine, with 52 million people and a quarter of the Soviet empire's industry, does not wish to be part of the union, what then? The big questions are what kind of a union, if any, will survive and who should the West deal with. Mr. Gorbachev last week brought the respected Eduard Shevardnadze back to his role as foreign minister to shore up the Soviet center, the "Union of Sovereign States" Gorbachev hopes for. Those hopes were trampled again Tuesday when the republics refused to endorse a treaty of confederation. The center seems increasingly impotent, unable to provide much but a forum for the distribution of Western aid. Gregor Yavlinsky, Gorbachev's ace economist, has yet another radical reform plan - but can a shaky center carry it out? So far, the US has taken the position, through the International Monetary Fund, that it will deal only with the center in aiding the desperately impoverished Soviet territories. That policy must be reviewed after Sunday's vote in the Ukraine. The US does not want to be seen as coercing the Ukraine into a union it rejects. As in the early stages of the Yugoslavia breakup, however, the US should strongly back a loose confederation which keeps Russia and the Ukraine in a constructive partnership. A giant new country in Eastern Europe with an army larger than the German Wehrmacht will not go over well in Russia. As with the Serbs in Croatia, there is the issue of the rights of ethnic Russians in the Ukraine. Direct aid to the Ukraine, dismantling of its nuclear arsenals, how to recognize it, what stipulation to make regarding minority rights - all require significant new diplomatic channels.