AFTER a political journey that began with a dramatic resignation and a warning of coming dictatorship last December, and lead to his presence on the barricades of resistance to the hard-line coup in August, Eduard Shevardnadze is back in his post as Soviet foreign minister.The decision to accept Soviet President Mikhail Gorbachev's nomination was "very hard, harder than the resignation," the silver-haired former Georgian Communist Party chief told the daily Komsomolskaya Pravda. Both Mr. Gorbachev's offer and Mr. Shevardnadze's acceptance seem motivated by the same desire - to avert the disintegration of the Soviet Union and to enlist the aid of the West in that seemingly desperate cause. "I couldn't afford to remain a passive onlooker," Shevardnadze told the independent Interfax news agency yesterday. "We have to go the barricades in order to save the world, since an unstable Soviet Union is a major threat to peace on the whole planet." Many here believe their future is being played out nearby in the brutal Yugoslav civil war, which erupted from the Serbian resistance to Croatian and Slovenian aspirations for independence. "Now we are swiftly approaching the Yugoslavization of our political life," St. Petersburg Mayor Anatoli Sobchak told Western reporters on Tuesday. "If we do not draw the appropriate conclusions from the Yugoslav tragedy, then we will be faced with the same problem, but on a much larger scale. National priorities, national emotions overcome everything, even common sense. They even kill the normal instinct for survival of people." Mr. Sobchak shares the concerns expressed by numerous political leaders here of social eruptions in coming months. He compares the economic situation to that in Germany in the 1920s, worrying that Russia, too, faces the danger of a "national socialist" movement emerging from such deprivation and chaos. The readiness of Germany to provide aid to the Soviet Union comes from the fact that "Germany itself has lived through this situation," the popular politician believes. In contrast, Sobchak observes somewhat bitterly, "the leaders of the Group of Seven (G-7) and the European Community constantly say they are ready to grant economic aid ... but in reality nothing is being done." He appealed for food aid as crucial to avert political chaos. On Tuesday, the United States approved a $1.5 billion package of agricultural credits and grants, about half of what Gorbachev had requested. In meetings with republican officials that began here Monday, deputy finance ministers from the G-7 offered a package of measures to avert the serious Soviet debt crisis. According to various reports, the Western financial officials will defer repayment, until the end of 1992, of a portion of the debt (the principle, not the interest) equal to about $6 billion, plus an emergency loan of $1 billion to service short-term debts. This package, however, falls far short of what Russian Deputy Prime Minister Yegor Gaider, head of the Russian delegation to the talks, feels is necessary. Mr. Gaider told a small group of Western reporters that the delegation seeks to reschedule payments on the principal of some $40 billion due for payment through 1993. The Russian official revealed official data showing total debt of $84 billion, far higher than previously estimated. Of this, $4.5 billion must be repaid by the end of this year, $16.5 b illion in 1992, and $18.5 billion in 1993. Sobchak and Shevardnadze, who both joined a group to advise Gorbachev after the failed coup, belong to a circle of Soviet liberals who are more comfortable with the Soviet leader's vision of continued political union. They back Gorbachev's untiring effort, renewed in recent weeks, to get the 12 republics of the former Union (without the three now independent Baltic states) to sign a treaty to form a "Union of Sovereign States." But many here see that effort as doomed by the determination of many republics, including the powerful Ukraine, Moldavia, Georgia, Armenia, and Azerbaijan, to pursue political independence. All of the republics except Georgia have signed the treaty to form an economic community. But in practice even that commitment has so far been nominal. That was clear from the G-7 meeting itself where only nine of the 12 republics agreed to observe a memorandum on collective responsibility for repayment of the Soviet debt signed Oct. 28. (Uzbekistan, Azerbaijan, and the Ukraine opted out.) More seriously, the republics have been unable to agree on a budget for the last part of this year for 1992. At a meeting of republican leaders yesterday, the Russian government voted down an emergency budget for the fourth quarter of this year, with a deficit of 110 billion rubles for that period alone. Gorbachev told the Soviet parliament yesterday that the overall Soviet deficit will amount to 205 billion rubles. The Russian government fears this spending will add to inflation at a time when it is on the verge of implementing liberalization of all state-controlled prices. "For me as a representative of a nation that was an empire builder, of course the destruction of the union is some kind of tragedy," says Gaider, "but I have to accept it as a reality."