AS he arrives in the Ukraine, George Bush steps onto fresh territory for an American president. Not only is it new ground geographically - no leader of the United States has set foot in a Soviet republic other than Russia since Franklin Roosevelt - but, more significantly, the political scene is without precedent.Last week Soviet President Mikhail Gorbachev nearly closed his deal with nine republics on the future shape of their union. The Ukrainians, however, held out. Joined by the Russians under Boris Yeltsin, they refused to OK a treaty that would allow central authorities in Moscow to levy a federal tax. The revenue needed by the "center," they asserted, would be determined by the republics. Unlike the secession-bent Baltics, the Ukraine has slowly, quietly been changing from one of the most pliant parts of the old union into an unbending champion of republics' rights. No republic, with the exception of Russia, has a bigger economic stake in throwing off the shackles of stifling communist management. The Ukraine is potentially an agricultural and industrial powerhouse. When nationalism started to surface three or four years ago, and ties to the Soviet center began to fray, common wisdom held that when the process reached the Ukraine, the game was up for the USSR as Westerners had known it since 1917. In essence, that prophecy is being fulfilled. President Bush will address a Ukrainian parliament that, while still nominally controlled by communists, is alive with nationalism. Many Ukrainians will see President Bush's trip to their capital, Kiev, as confirmation of the republic's new, more independent status. The US president, for his part, will have to maintain the value of Gorbachev's central role even as he acknowledges the Ukrainians' right to chart their own course. This summit, and particularly the stop in Kiev, could mark a sharp turn in US-Soviet relations - and the advent of US-Ukrainian (or Russian, or Lithuanian, or Armenian, etc.) relations.