BY THE TIME Mikhail Gorbachev warned last week that a breakup of the Soviet Union could lead to "bloodshed and civil conflict," it had become widely acknowledged that if reform is going to work -- and if a workable Soviet Union is to be preserved -- it must be led by a center-left coalition. And what had become abundantly clear was that this coalition would be headed by Gorbachev and Boris Yeltsin. Their longstanding rivalry has evolved into a partnership, forged by necessity.

No one, perhaps, is more aware of this than Gorbachev who, finding his back against a wall, made a decisive, and public, move away from the centrist stance he has held for most of his tenure. The shift towards the more progressive positions of Yeltsin, the president of the Russian federation, has so far cost Gorbachev one ally and some control over the Soviet provinces.

But it has gained him no less than the chance to salvage his job and his country.

To be sure, some personal bitterness remains; when Gorbachev told Soviet lawmakers last week that "there are forces that don't want to normalize the situation," some took that as another criticism of Yeltsin. But last week's meeting of the Supreme Soviet also provided evidence that a period in which both Gorbachev and Yeltsin tried, separately -- and without success -- to address the Soviet Union's economic woes has ended.

Gorbachev's moderate policies may have kept the center together, but at a terrible cost: dire shortages of consumer goods (such as bread), with few incentives for increasing production. Yeltsin, for his part, decided to barrel ahead in the Russian Republic with an ambitious program to turn the economy around in 500 days. But Yeltsin understood that he had no chance of success without support from the center. In other words, Gorbachev and Yeltsin not only depend on one another, they have come to recognize how necessary each one is to the other.

"When it comes to reform, the two of them depend on one another," Alexander Tsipko, an adviser to Gorbachev, said in a recent interview. "Yeltsin can afford to be more freewheeling because he has no real power. But it is also true that Gorbachev needs someone more aggressive to needle him along."

Important differences between the two leading Soviet politicians remain. Gorbachev, who still heads the Communist Party, is resisting demands to formalize a power-sharing arrangement with the influential circle of non-party members, which includes Yeltsin. And the differences between the two over a proposed union treaty are significant: Yeltsin is openly chiding Gorbachev for not yielding enough control to the leaders of the 15 republics, advocating far greater autonomy than Gorbachev is willing to concede. But some republics are demanding even more independence than Yeltsin is asking, and Yeltsin is the only republic leader with enough stature to shape the negotiations.

The two are closing the distance between them. Criticized by angry Soviet parliamentarians last week for favoring temperate solutions to the nation's woes, Gorbachev gave in to some of their demands. The most important of these was dropping Prime Minister Nikolai Ryzhkov, a close colleague who had also become a symbol of the middle-of-the-road economic course. Gorbachev also agreed to overhaul the leadership of the military and the party apparatus.

By agreeing to such concessions, Gorbachev made major strides in the direction of Yeltsin and the more radical policies he advocates, such as accelerating Moscow's transition to a market economy and loosening its control over the rebellious outlying republics. As one American diplomat observed, "Gorbachev has now crossed the barrier that has divided him from Yeltsin since the beginning. He cannot go back. They are on the same side of the political fence."

But if the union between Moscow and the republics is going to survive, even closer cooperation between Gorbachev and Yeltsin will be needed to preserve it -- and even that may not be enough to save the crumbling Soviet empire from the crash landing to which it now appears headed. Whether either of them has a workable plan for slashing the out-of-control Soviet deficit -- or addressing other such pressing problems -- is unclear. Still, their resolve and commitment to overcoming past differences will do much to determine their success.

In the West, it has become commonplace to describe Gorbachev and Yeltsin as rivals, but in fact both leaders have understood for some time that their "rivalry" depends upon a degree of mutual support. Some of the support between the two came to the fore in Moscow last July, at the 28th Party Congress. At that conclave, an extraordinary event took place: When an organized attempt by conservatives threatened to disrupt the meeting, Yeltsin moved in quickly to defend Gorbachev.

In a keenly timed and carefully worded speech, Yeltsin warned that the party must either adopt a more progressive platform or perish. Aware of the conservatives' plan to take control of direction of the party and abandon the reform drive, Yeltsin outlined the alternatives facing the congress of communist leaders with these words:

"Either there will be a party of the apparatus that inevitably will break up and sooner or later will leave the only group of political forces that amounts to anything. Or a renewed party will be transformed into a union of democratic forces with some hope of keeping an active role in perestroika."

Yeltsin's speech was not a spontaneous oration but part of a well-thought-out plan to protect Gorbachev and his reforms. At first, apparently made aware of a conservative plot to use the congress to oust the reformists, Yeltsin had sought in vain to get the meeting postponed. Failing that, as the anti-Gorbachev mood was rising across the floor of the congress, Yeltsin stepped in. Using his great oratorical skills, the burly, commanding Russian leader effectively warned delegates to the congress of the choice it was facing if the conservatives were allowed to hold sway. In a threat that sent shivers across the Palace of Congresses, where the gathering was held, Yeltsin warned that if the party apparatus won its battle for control, there would be be nothing left to do but "begin the struggle to haul the party leaders of all levels into court for the damage they have done to the country."

The speech turned the mood of the congress abruptly in Gorbachev's favor. Afterward, conservative bids to wrest power -- such as hardline politician Yegor Ligachev's run for the post of deputy general secretary of the party -- were roundly thwarted. In the end, Gorbachev managed to convince the congress to endorse his reforms and approve his choices for party leadership positions.

Yeltsin's move was far from altruistic. In the short run, he could have profited from the defeat of Gorbachev. But such a turn would surely have brought upheaval in the party and the country -- a development that even the enormously popular Yeltsin might not have survived. In saving Gorbachev, Yeltsin was also acting to preserve order in the party and the Soviet political system. And yet, quickly grasping the declining influence of the party in national politics, Yeltsin made a surprise second speech to the congress -- announcing his resignation from the Communist Party.

Just as Yeltsin moved closer to Gorbachev, Gorbachev moved closer to Yeltsin. Though clearly shaken by the Yeltsin's decision to leave the party, the Soviet president publicly declared his intention to work closely with non-communists, such as Gavril Popov and Anatoly Sobchak, the respective mayors of Moscow and Leningrad. Earlier this fall, Yeltsin and Gorbachev entered into protracted negotiations over the 500-day emergency program for economic reform. Those negotiations ended in disagreement, with Gorbachev deciding not to adopt the program, but the two and their advisers have since maintained close contact over the course of reform.

There are, to be sure, strong personal motives behind the alliance. By aligning himself with Yeltsin, Gorbachev is clearly seeking to bolster his own sagging public ratings. By forging closer ties to Gorbachev, Yeltsin is preserving the Soviet presidency in the event that he inherits it himself. The more junior politician also needs the authority Gorbachev commands by virtue of his international stature, the offices he holds and his political acumen. "Even if he is unpopular," a U.S. official said in an interview, "Gorbachev still holds sway over important bodies -- like the military, the security organs and the party. Yeltsin, being a creature of the Soviet system, is aware that he cannot succeed without some cooperation from those bodies." The future of the Gorbachev-Yeltsin alliance -- and its direction -- is unclear, though the two are already negotiating the terms of their pragmatic arrangement. In the past week, they staked out their opening positions: Gorbachev created a new body, the Federation Council, that would give Yeltsin -- and his 14 fellow republic leaders -- broad powers in guiding the Soviet Union away from the brink of economic collapse. Leaving the precise terms of the arrangement loosely outlined, Gorbachev left for Paris to meet with European leaders.

Yeltsin objected strongly to the loose way the council's role was posited. In an interview with the Soviet Interfax news agency, he demanded that Gorbachev define and strengthen the powers given to the republic leaders.

The importance of the council will undoubtedly grow over the long term. As a body composed of regional leaders, it will almost certainly become a forum in which the different republics will haggle over how much sovereignty they will get in the future Soviet federation.

Nothing is more pressing, though, than how Gorbachev and Yeltsin will address the Soviet economic crisis. The problems, ranging wide, are highlighted by a burgeoning deficit, an overvalued ruble and a complete absence of an investment policy. So far neither Gorbachev nor Yelstin has come up with a feasible plan for tackling any of them. And even though the two have pledged cooperation in this area, Gorbachev is still apparently reluctant to endorse the fast pace of reform that Yeltsin advocates.

Asked why Gorbachev opted against the 500-day economic reform program, one of his key advisers said the Soviet president feared it would bring about his downfall. "I was as surprised as anyone that he canceled the program," Abel Agyanbegyan said in a recent interview, "I think he thought it would mean the end of our administration."

But over the past 5 1/2 years, Gorbachev has proven extraordinarily resilient in the face of constant and bitter opposition, defying predictions of his imminent political demise. And though has evolved as a politician, he has never shown signs of abandoning the reform drive that has made his name a worldwide household word. For him, alliance with Yeltsin is clearly a way to keep both his career and his reforms alive. Yeltsin, too, is a model survivor. It is easy to forget now that he was ousted from the Soviet leadership just three years ago but has bounced back from political limbo to assume the second most powerful position in the Soviet Union.

Whatever course they take, neither Yeltsin, Gorbachev -- nor the reforms -- are likely to go away soon. "All three are somehow intertwined," Tsipko said. "You might even say they are tragically linked."

Gary Lee, a Washington Post reporter, was Moscow bureau chief from 1985-88. Guilietto Chiesa, a Moscow correspondent for La Stampa, contributed substantially to this article.