MOSCOW -- Falling back on his eroding but still powerful political base, President Mikhail S. Gorbachev yesterday called on the Communist Party to fight for the signing of a union treaty to preserve the Soviet Union.
"Today before the party stands the task of supporting in every way the creation of a powerful political movement for the preservation and renewal of the union," Mr. Gorbachev told a meeting of the party's 250-member policy-making Central Committee. "This is a decisive test for the Communist parties of the republics as well.
"Our country has become an integrated organism," he said. "Breaking ties built up over decades and in some cases centuries, and then trying to restore them, would do incalculable harm to all our peoples. Several generations would have to pay a heavy price."
Noting that 60 million Soviet citizens -- approximately 20 percent of the population -- live outside their national republics, he said secession of republics would set off disastrous ethnic migration.
"To put that enormous human mass in motion would bring on a cataclysm that would catastrophically affect the life not only of the country but of the whole world," he said.
Faced by a wave of declarations of independence or sovereignty among the 15 republics, Mr. Gorbachev has proposed a new treaty of union to replace the original treaty that created the Soviet Union in 1922.
But leaders of Estonia, Latvia, Lithuania and Georgia have said their republics insist on full independence and will refuse to sign any union treaty. Several other republics have expressed doubts about the treaty.
Some speakers insisted that socialism be retained as the official state ideology and the word "socialist" kept in the name of the renewed union, said Alexander V. Buzgalin, who attended the closed meeting, in a phone interview.
To attract the backing of rebellious republics, Mr. Gorbachev has dropped any mention of socialism from his proposed draft union treaty and recommended the name "Union of Sovereign Soviet Republics" in place of today's "Union of Soviet Socialist Republics."
Mr. Gorbachev remains general secretary of the 19-million-strong Communist Party despite the repeal last spring of the party's legal monopoly on power and despite many suggestions that he should quit the post and focus exclusively on the presidency.
Mr. Gorbachev evidently believes that for now, he cannot rule without the party post. Russian leader Boris N. Yeltsin and the nationalist leaders elected in many other republics have deprived the Soviet president of a broad popular base, leaving him only the party as a political foundation.
In addition, despite its falling membership and plummeting prestige, the Communist Party remains, as Mr. Gorbachev pointed out in his speech, "the only political force on the scale of the entire union."
Thus, Mr. Gorbachev is at once, in the words of a commentary in the outspoken television program "Vzglyad" (View) last week, leader of the Communist Party and hostage to it.
In his speech to the plenum, the official text of which was read over Soviet radio last night, Mr. Gorbachev blamed nationalists for the threat to the Soviet Union's integrity:
"I'll say directly, there's no more serious danger in the country than extreme nationalism and the whipping up of ethnic hatred."
He said a battle for control of wealth is behind disputes between republics and the union and the resulting "war of laws." He clearly had in mind Mr. Yeltsin's Russian Federation, which is now negotiating with the Soviet government for control of Russia's enormous oil, natural gas, gold and diamond reserves.
Mr. Gorbachev rejected arguments from Mr. Yeltsin and others that an economic agreement, combined with separate political treaties between pairs of republics, could supplant the union treaty.
He said republics and their people benefit from their common economic market and international clout offered by the union's superpower status. "I do not think any republic would win by giving up this indisputable advantage," he said.
Mr. Gorbachev defended what he called the "achievements" of the union and rejected accusations that the Soviet Union is the old Russian empire in disguise.