After months of hesitation toward Moscow, President Bush finally decided last fall that the time had come to engage Soviet President Mikhail Gorbachev. Bush ended the Malta summit declaring that he and Gorbachev stood "at the threshold of a brand new era of U.S.-Soviet relations."

Tomorrow, as Bush ushers Gorbachev into the Oval Office, that new era will dawn, but not without complications for Bush that were unforeseen six months ago. Instead of an exhilarating grand entrance by both leaders into the post-Cold War world -- which seemed in prospect a few months ago when the Soviets were rapidly moving toward the West -- Bush is now faced with a Soviet president under pressure at home who appears less flexible or creative than he did at Malta.

For Bush, the first summit of his presidency in the United States has been framed by seemingly unanswerable questions about Gorbachev's future, his troubled nation and the shape of Europe as it enters the next century, according to high-ranking administration officials and private analysts.

While Gorbachev is besieged at home, Bush comes to the summit reinforced with exceptionally strong political support, perhaps greater than any American president since John F. Kennedy at this point in his term. But there are also risks for Bush should he fail to seize the moment. According to intimates, Bush believes he must translate the revolutionary events that occurred abroad during his first year in office into real accomplishments that have so far eluded him. These include a strategic arms treaty -- a major piece of unfinished business from the last decade -- and achieving the unification of Germany within the Western alliance, a major challenge of the new decade.

"The quick pace of 1989 gave the president the advantage of looking like he was the right man for a time of tremendous change and opportunity," said a long-time Bush adviser. "Now, 1990 is the year he must prove he can meet expectations."

But because of Gorbachev's many problems, Bush has been forced to lower his sights. "I don't want these two gigantic ships to pass in the night because of misunderstanding," he said last week in a more modest description of his hopes for the summit.

In the opening months of his presidency, Bush seemed uncertain about Gorbachev. He set the Soviet leader the challenge of passing a series of tests to qualify for U.S. cooperation. Gorbachev was dismissed as a showy "drug-store cowboy" by the White House spokesman, Marlin Fitzwater. Some of Bush's lieutenants openly questioned whether Gorbachev would survive, and warned against gambling too much on his success. Meanwhile, Democratic congressional leaders criticized Bush as too reactive and timid in responding to the momentous events unfolding in the East.

After many months Bush responded by declaring his support for Gorbachev's program of perestroika, or restructuring, and at the Malta summit he offered the Soviet president a host of new U.S. initiatives to demonstrate his support. Later, impressed by the swift collapse of hard-line communist regimes in Eastern Europe without interference from Moscow, Bush went further, saying he wanted Gorbachev personally to succeed and calling the Soviet president a "tremendous statesman."

Now, against a backdrop of panic-buying sweeping the Soviet populace, Gorbachev's high-wire act is a profound concern in the White House. But this time there is an important difference: The discussion is not about whether Bush should support him, but about how the United States can contribute to Gorbachev's staying power. Some U.S. officials said that, despite whatever doubts they harbored about Gorbachev early last year, they now shudder at the thought of a Soviet Union suddenly without him. And Bush seems committed more than ever to Gorbachev as the agent of stability and reform in the collapsing world of communism.

"Today, the enemy, in my view, is instability, unpredictability . . . lack of confidence in each other," Bush told West German television last week. A senior White House official said yesterday that it would be a "hazardous game" to wait for a new Soviet leader. "We should not load problems onto Gorbachev. It's difficult for me to imagine it would be easier under almost any other prospective Soviet leader," the official said.

Bush "knows the downside of chaos," said Rep. Jim Leach (R-Iowa). "To make deals as well as shape the context of policy, you have to have a degree of stability on the other side as well as your own. Bush has to be cautious about undercutting the ability of the other side to make decisions. I think he instinctively understands that."

Ed A. Hewett, a senior fellow at the Brookings Institution who last week briefed Bush on the Soviet economy, said Bush understands that Gorbachev "is the only national level leader capable of giving you a fairly orderly transition. You know you've got a Soviet Union moving toward a confederation and more economic and political democracy. What you don't know is which of several paths you're going to take to get there. Some could be enormously bloody, some orderly. Gorbachev has that second path in mind. I don't know of anyone else of a national status in the Soviet Union who would have a ghost of a chance. We're betting on Gorbachev, warts and all."

"We have no way of knowing about what the alternative to Gorbachev would be," concurred a senior administration official. "You stick with someone who has a track record. He has a track record of reform. He has not gone back on his word in Nicaragua or Afghanistan. Why should we do anything to upset him? From the president's point of view, he's got a known quantity he can deal with."

The test of this relationship may come in the summit discussions over Germany. Gorbachev has said he wants a reunified Germany to be neutral and out of the North Atlantic Treaty Organization. At the same time, the East-West talks on reducing land armies and tanks in Europe have stalled, apparently due to Soviet uncertainty about the future of a powerful unified Germany and the kind of European security apparatus that will arise.

"Germany is the big prize," said a high-ranking official. "That is the hidden agenda. The large goal is to get the Soviets out of Eastern Europe and keep Germany tied to the West."

According to administration officials, Bush has still more "flexibility" in mind as he attempts to coax Gorbachev this weekend to accept a unified Germany in NATO. But Gorbachev may not be persuaded by proposals that only postpone the day when Germany is fully independent, united and a member of the Western military alliance. And there are analysts who question whether Bush is prepared for a next step that might appeal to Gorbachev more, such as the creation of a new European security structure. Bush has talked vaguely about a political role for the 35-nation Conference on Security and Cooperation in Europe, but the United States has offered little leadership toward constructing a more ambitious security organization.

"This is the time to do that," said defense analyst Barry Blechman. "The Soviets can't accept that all that's gone on is that NATO's boundaries get extended. Yes, they can accept NATO as a continuing organization, a consultative organization; it doesn't have to disappear. But it has to be transformed. We have to be willing to build a new security framework in one organization that's charged with overseeing the peace."

"Just pressing Gorbachev" to have Germany remain in NATO, he added, "doesn't mean we have a vision" about the future of Europe.

The administration consensus is that Bush can offer Gorbachev little serious help on the economic front. The most visible manifestation of U.S.-Soviet economic cooperation, the trade agreement, remains in limbo as long as Moscow keeps Lithuania under an economic embargo. Bush and other allied leaders have shied away from any more ambitious plan to pump new capital into the Soviet Union until the difficult road to a market economy is well underway.

Hewett said, "I think the administration is really trying to figure out if there are other things they could be doing." U.S. officials are "acutely aware of the limits," he said, "and know in the end they won't make the difference." But, he added, "They would not like to have it said a few years from now that they sat on their hands, and it would be a shame if they did."

Staff researcher Bruce Brown contributed to this report.