THE Soviet coup early yesterday morning strikes a dangerous blow to the deepening American relationship with the Soviet Union.That relationship had become strongly centered on President Mikhail Gorbachev who, although no longer the banner-carrier for reform, succeeded until yesterday in holding the Soviet Union's tumultuous factions together. Although fully aware that Soviet leader Mikhail Gorbachev was walking a precarious tightrope between conservatives and liberals who criticized the pace of his economic and political reforms, Bush administration officials were caught completely off-guard by news of Gorbachev's ouster. President Bush was awakened and informed of the coup at his summer house in Kennebunkport, Maine, just before midnight Sunday. In the morning, he called it a "disturbing development" that could have "serious consequences" for relations with the US. Bush announced all US aid to the Soviet Union was on hold. Bush has frequently insisted that US relations with the Soviet Union do not depend on Gorbachev. Even so, US officials have credited the former Soviet leader with unusual skill in following a middle line between liberal revolution and the kind of conservative reaction that swept him from power yesterday. Gorbachev's departure occurs at a high point in US-Soviet relations. Just last month Bush and Gorbachev met in Moscow where they concluded a strategic arms reduction treaty. Days earlier, Bush and the leaders of six other industrial democracies pledged further efforts to speed free market reforms in the Soviet Union. The personal chemistry and trust that has developed between the two leaders over the past couple of years was apparent at the Moscow summit. In Moscow, Bush underscored Gorbachev's importance as the Soviet Union's strongest link to the West and its economic resources by offering the Soviets everything he could, without spending American money. Most significantly, Bush finally offered the Soviets most-favored-nation trading status. In the early hours after Gorbachev's ouster one concern here is that such reforms, halting as they have been, could be reversed. A more pressing concern is that opposition to the takeover could produce violence and political instability, especially in several restive republics which have been seeking to break away from the Soviet Union. The risk that the hard-liners now in power will return to a cold-war confrontation that threatens Europe and the West is not rated high. The Soviet Union still has over 10,000 nuclear warheads mounted on intercontinental ballistic missiles capable of reaching the United States. Its ground forces, however, have largely withdrawn from non-Soviet Europe. Further, most American analysts predict that the hard-line right will be too consumed with the task of consolidating its power within the Soviet Union to try to expand its sphere of influence abroad. The Soviet economy, also, is in too much of a shambles to support military adventures. Yesterday's coup occurred just as a new union treaty designed to redistribute power within the country was about to be signed by Gorbachev. That treaty, which would grant more autonomy to the republics, is almost certain to be scrapped amid threats by the new leadership to suspend the parliament of any republic pressing for secession. Any massive Soviet intervention to block secession, for example in the three Baltic republics, could present the US with the difficult choice of whether to intervene. In addition to the strategic arms treaty, now awaiting Senate ratification, Sunday's coup also places a treaty on conventional forces in Europe, signed in November, in doubt. Gorbachev's departure also threatens joint initiatives which could help bring peace to regions of the world that were once theaters of intense superpower competition. A major question mark now looms over plans to convene a comprehensive Middle East peace conference under joint US and Soviet auspices. Israel may be less inclined to participate with a hard-line Soviet sponsor. Analysts will be watching, meanwhile, to see if Syria is influenced by the changes in Moscow to revert to its traditional hard-line policy towards Israel and to eschew US-led peace efforts. While in the Ukraine just over two weeks ago, President Bush stressed that American policy was not focused on personalities, namely Gorbachev. But rather it supported the principles of democracy and reform, whether they came from Moscow or the restless republics. But the program Bush promoted was clearly that of Gorbachev, as the man in the middle of Soviet politics. With the loss of Gorbachev to a hard-line coup, Bush policy may be proven right even as it fails. Bush's critics accused the President of holding back stronger support for the independence movements in the republics. The greater threat to stability - if the early course of this coup is borne out - remained on the Soviet right.