MOSCOW, DEC. 31 -- Describing 1990 as one of "the most difficult years in our history," President Mikhail Gorbachev predicted today that the coming year would decide the fate of the Soviet Union as a multinational state.

"For all of us Soviet people, there is no more sacred cause than the preservation and renewal of the union in which all our peoples can live well and freely," Gorbachev said in a somber New Year message to the nation. "Perhaps we now feel as never before that we simply cannot live partitioning ourselves off from one another."

Nearly six years since he became Soviet leader and launched his perestroika reform program, Gorbachev implicitly acknowledged tonight that many Soviets hold him at least partly responsible for the country's deepening political and economic crisis. He said that "mistakes" and "inadequacies" committed by the "country's leadership" had compounded the inevitable problems of attempting to bring change to such a gigantic, ethnically diverse state.

Gorbachev's popularity has plunged over the past year as millions of Soviet citizens suffer the degrading experience of waiting in long lines for food and other basic consumer items. The Soviet leader also has been accused of indecision because of his repeated hesitation over the choice of a coherent economic package to pave the way toward creation of a market economy to replace the inefficient, Communist-installed central-planning system.

Gorbachev was elected the Soviet Union's first president by the national legislature in March and has accumulated vast constitutional powers since then, including the right to rule virtually by decree. But as his political powers have grown, his real authority in the country seems to have declined, and many presidential decrees are ignored.

Tonight, Gorbachev promised to use his executive powers to "move forward on the path of democratic transformation, strengthen order, law and discipline and defend human rights." He said it would be possible to achieve a "turning for the better" over the coming year, given an atmosphere of "civil and inter-ethnic accord, responsibility and discipline, diligent work and humaneness in dealing with each other."

Hard-line Communist Party politicians and senior army officers have urged Gorbachev to use his new powers to crack down on the most troublesome of the 15 Soviet republics, most of which are clamoring for increased autonomy or outright independence. Some have argued that he should introduce direct presidential rule in the Soviet Baltic republics, Moldavia, and the three turbulent republics of the Transcaucasus region, suspending democratically elected governments in those areas.

During the last session of the Congress of People's Deputies, the country's supreme legislative authority, Gorbachev made clear that he intended to take a tougher line with the republics. His more decisive approach seems to have borne fruit in Moldavia, where the ethnic Romanian majority is embroiled in a political and linguistic conflict with the ethnic-Russian and Turkic minorities.

After receiving an ultimatum from Gorbachev to reconsider controversial legislation within 10 days, the Moldavian legislature backed down this weekend. Many observers expect the Soviet leader to use similar tactics against the Baltics, where independence movements are further advanced than in other Soviet republics.

In an exchange of New Year's greetings with President Bush, Gorbachev made clear that there would be no major changes in Soviet foreign policy, despite the recent resignation of foreign minister Eduard Shevardnadze. He said he hoped the international community would find strength to overcome the "serious obstacle" to a new world order posed by the Iraqi invasion of Kuwait.

Turning to his meeting with Bush in Moscow this February, Gorbachev said he hoped it would strengthen "the sound foundation of our relations." For his part, President Bush used a videotaped New Year's message to the Soviet people to salute Gorbachev's efforts at domestic reform and the Soviet leader's support of actions in the Persian Gulf.

"I applaud, the world applauds, the decisive action of the Soviet Union in strongly opposing Saddam Hussein's brutal aggression in the gulf," Bush told a Soviet national television audience. "I also want to applaud the Soviet Union for the important steps you've taken in building a new society, for the determination with which you are pressing forward with difficult political and economic reforms."

"It's an arduous journey, but one well worth making, for it is a path that leads to a brighter future for your nation," Bush said.