NEWS ANALYSIS : Two Presidents Enjoy Summer ‘Romance’--but Fall Is Near : Diplomacy: The air is filled with the boundless energy of a ‘grand alliance.’ But will the moribund Soviet economy turn it to so much dross?

For the span of this week’s superpower summit, the towers of the Kremlin almost sparkled under clear blue skies. A gentle breeze swept the air clean. Lovers promenaded in Gorky Park. And the uplifting spirit of summer seemed ready to last forever.

Just so, George Bush and Mikhail S. Gorbachev ended their meetings Wednesday with plans for a “grand alliance” of boundless superpower cooperation. And, building on the successful cooperation in the Persian Gulf, they immediately joined forces in a new attempt to drive the Middle East’s implacable foes to the peace table.

Yet, even as the Russian summer is only a brief respite between gray winters--and as the summit’s air of hope contrasted with the weary and resentful faces of Moscow’s people as they waited in endless lines to buy even the simplest needs--so a grimmer reality lies in wait for Presidents Bush and Gorbachev.

If they cannot make headway against the intractable problems of the moribund Soviet economy--and make it soon--their vision of an irresistible partnership in shaping a new world order will likely slip away in the months ahead like the leaves of the birch trees on the Lenin Hills.

“I heard about the summit meeting, but our economic problems are more important,” said Nina Kudryavtseva, 40, an ambulance medic walking wearily home with two heavy bags of hard-won groceries. “I’m afraid our situation is hopeless. I have no faith this will ever end.”

“The summit might be a good thing because if there is less military spending, the government might spend more money on social needs,” said Nikolai I. Volkov, a 67-year-old retiree and World War II veteran with a row of frayed campaign ribbons on his coat. “But then, probably not. The bureaucrats will take all the money for themselves. . . . In the end, the summit will change nothing.”

It is voices such as Volkov’s, lost in the pomp of summitry, that represent the true test of what Gorbachev and Bush together can accomplish.

As it has evolved with startling speed since Gorbachev set the wheels of change in motion, the U.S.-Soviet relationship has come to have two new focal points:

* International affairs, once the stuff of superpower conflict and now an opportunity for surprisingly easy cooperation and great potential for influencing events around the world.

* Soviet domestic policy, an area that was virtually impervious to outside influence for 70 years but is now crucial to everything Bush and the Western allies hope to achieve with Moscow.

The rhetorical proclamations by Gorbachev of a new kind of U.S.-Soviet relationship, centering on American efforts to help with Soviet reform, turned out to be true.

“For the first time . . . we emphasized the shape of our economic relationship and how to work together in that important area,” he said.

“The next challenge,” Bush agreed, is “furthering economic reform in the U.S.S.R. and seeking to integrate the Soviet economy into the international system.”

There was less apparent disagreement in Moscow this week than at any summit meeting in memory. On traditional areas of conflict, Bush and Gorbachev blandly papered over what differences remain--in arms control, Soviet aid to Cuba and Soviet rule over the Baltic republics.

Instead of mutual suspicion, they emphasized an unprecedented pair of joint policy statements on two of the world’s thorniest issues: the Arab-Israeli conflict, once an arena for Cold War gamesmanship, and the breakup of Yugoslavia, Europe’s most explosive crisis.

The language of the two joint statements may have seemed bland, but the point was clear to all: The two superpowers are now working as partners in international peacemaking.

And they declared their intention to continue joint diplomacy--through trips by Secretary of State James A. Baker III and Foreign Minister Alexander A. Bessmertnykh, through work at the U.N. Security Council and through behind-the-scenes diplomatic spadework.

The idea, U.S. and Soviet officials said, is to combine the weight of the superpowers so that each nation gains more impact than it would acting alone.

In the Middle East, for example, Israel can resist the joint U.S.-Soviet effort only at the cost of being identified worldwide as deliberately rejecting peace.

As Gorbachev said of the joint statement on the Mideast: “This is a very important result of our joint work, and I think that the fact that this position will be publicly announced will have serious influence on this process. We consider that it is in a decisive stage, and we should not lose . . . this window of opportunity.”

Five years ago, after four decades of fighting the Cold War through proxy conflicts across the Third World, the Soviet Union and the United States began working toward negotiated solutions of some regional problems.

But their first stage of efforts, on such conflicts as Afghanistan, Angola, Nicaragua and Cambodia, concentrated largely on ending wars that the superpowers themselves had helped to start.

That pattern changed last August with Iraq’s invasion and seizure of Kuwait. The United States and the Soviet Union immediately forged a joint approach that condemned Iraq’s aggression and, warning of dire consequences, demanded its immediate and unconditional withdrawal.

Against skeptics’ predictions, the alliance held. Although the Soviet Union committed no troops to the Gulf War, it did back the military campaign that drove the Iraqis out in February.

“It was a test, and we passed,” a Gorbachev aide commented this week.

Bush called it “the crowning proof that we are overcoming old Cold War animosities.”

And despite jibes about the Soviet Union as the “former superpower,” it remains a substantial force. It covers one-sixth of the Earth’s land surface, touching Europe, Asia and the Middle East. It retains the world’s largest armed forces and a huge nuclear arsenal. It is one of the world’s largest arms suppliers. And it still has influence in some areas where the United States encounters hostility.

But along with the promise of the new grand alliance, the United States suddenly has acquired quite a different interest in the progress of the Soviet Union’s political and economic reforms.

Washington very much needs Gorbachev’s survival, for he is the Soviet leader who has committed his country to the partnership with the United States. His ouster, at the least, would turn the Soviet Union inward for a period of what could be nasty political skirmishing--diminishing Moscow’s ability to work with the United States in global affairs.

The Bush Administration has also become openly committed to the Soviet Union’s survival as a state--and thus will oppose its disintegration through separatist moves, aside from those in the three Baltic republics.

And the United States, wanting Gorbachev’s reforms to succeed, has resolved to provide support--at first the technical assistance that was again pledged during this summit, but later, probably, the direct economic aid he will need for his country’s transformation.

Even on the international stage, the new alliance is not problem-free. Those being pushed may claim that the superpowers are bullying them. But the charge of a “superpower condominium,” initially raised nearly 20 years ago when Moscow and Washington first began to discuss regional conflicts, does not stick when they are acting as leaders of the international community as a whole.

In the Soviet Union, the policy has already been questioned by critics of what some Soviet analysts have dubbed Gorbachev’s “America First” policy. They fear that the Soviet Union is tying itself too closely to the United States and ignoring Europe, where they say it has greater interests.

Close cooperation with the United States will perpetuate the idea that the Soviet Union is a “superpower” with global interests, and this will cause the Soviets to continue to overstretch their resources, they warn. In this view, the Soviet Union should think of itself as one of the “great powers” of Europe and collaborate primarily with other European countries on the problems of the Continent.

Conservatives, particularly in the Soviet military, are also likely to oppose the grand alliance as another deviation from Marxism.

And over all these concerns, like the layer of grime that covers most of old Moscow, remains the aching fear that the Soviet reformers’ efforts to turn their country into a democratic, free-market system could collapse.

Then, the Soviet Union might not only be lost as a U.S. ally; it could well turn into a bear pit of civil strife and economic hardship that would dwarf any crisis since World War II.

For Soviet citizens on Moscow’s darkening streets Wednesday evening, such strife and hardship were clearly the overriding worries, summit or no summit. And so, in the months ahead, they will prove to be for Gorbachev and Bush.