On the eve of the 27th Soviet Communist Party congress, which opens here Tuesday, a mood of expectation has gripped this huge country and its capital. The hope is that party leader Mikhail Gorbachev, at the helm of a new post-Brezhnev generation, will use the occasion to launch an era of social and economic change.

The mood is hardly coincidental.

In nearly every public appearance in his 11 months in office, Gorbachev has given the event a plug. The Soviet leadership has employed propaganda, exhortation and example to build up this congress as a historic occasion. It has splashed the event with red-and-white placards plastered from Vladivostok to Yalta, countdown calendars posted in factories and schools from Siberia to Moldavia, nightly television news spots and a daily diet of congress-oriented features and editorials in Pravda, Izvestia and nearly every other official newspaper.

But while past party congresses have seen moments of high drama -- as in Nikita Khrushchev's denunciation of Joseph Stalin's reign of terror -- their substantive accomplishments have in large part consisted of the public unveiling of programs and decisions worked out in closed meetings during the preceding months.

Gorbachev, who pledged in October to bring about "transformations of a truly historic scale," has spent the ensuing months rushing out a game plan to achieve that goal.

The package of proposals, policy positions and personnel changes that he will place before the 5,000 congress delegates includes a revision of the 1961 Communist Party program, a sweeping plan for economic and social development, a near purge of senior party officials and at least 50 percent of Central Committee members, and an array of foreign policy initiatives, including a plan for world disarmament.

The programs' overall target date: 2000. Their apparent purposes: to rescue the moribund Soviet economy, restore the Soviet Union's reputation as the Communist vanguard and, not least, to revive sagging morale.

Can Gorbachev motivate the Soviet bureaucracy? Does he face resistance? Does he have a grand strategy?

Nearly a year after he came to power, the questions still dangle.

When the congress ends next week, they should not.

For Gorbachev, 54, the risks of failure, or of dashing high expectations, are great. He is widely regarded as the U.S.S.R.'s last hope of closing its technological gap with the West.

With the approach of the congress, Gorbachev has raised a flurry of expectations -- among economists, workers and artists and party members -- that he will use it as a forum to make good on his promise to usher in "truly historic" change.

Among western diplomats in this capital, confidence in the new Soviet leadership's will for substantive economic and social change is outweighed by skepticism that the modest measures outlined so far can accomplish the proclaimed goals: wide-scale development of industrial technology, a doubling of industrial output and a 150 percent productivity increase by the end of the century.

The growing conservative tone in official policy statements is a "bad omen," said one senior western diplomat. He added, "At best, they indicate mixed feelings in the party leadership about the need for and the means of reform."

The diplomat's evidence: rigid Kremlin positions on human rights, conservative editorials in Pravda and Izvestia, the persistent strength of remaining old guard Politburo members Dinmukhamed Kunaev and Vladimir Shcherbitsky and the strongly ideological statements made by Gorbachev in an interview this month with the French Communist newspaper L'Humanite and by second-ranking Politburo member Yegor Ligachev.

Soviet officials and western diplomats have begun to look to the party congress as a test of the new Soviet leadership's commitment to reform in both the economic and cultural spheres. Besides Gorbachev's major address Tuesday, the bellwethers among the 70 or so speeches scheduled for the nine-day session include those by Ligachev and Premier Nikolai Ryzhkov.

Much of the expectant mood has been brought on by Gorbachev himself.

By forcing several rewrites of Konstantin Chernenko's modest 1986-90 economic plan and replacing it with a sweeping 15-year blueprint for economic and social development, Gorbachev has raised hopes that he can succeed where his three ailing predecessors failed: to turn around sagging oil production and the low rates of agricultural output, to overhaul faltering industries and to improve the often shoddy quality of Soviet goods.

Said Rudolf Koltchanov, deputy editor of the labor newspaper Trud, in an interview: "The average Soviet worker hopes and believes that after the congress there will be more order in the management of our economy, greater impulse in agriculture and industry."

For the 18.5 million Communist Party members, Gorbachev has extended an even bigger carrot. He orchestrated a controversial rewrite of the 25-year-old party program, launched a campaign to rid the party of its image as a haven for the privileged, aging and corrupt, and forged a foreign policy that embraces renewed ties with the United States and its allies, as well as with the Third World. Last month, Gorbachev rounded off a year of image building at home and abroad with a proposal to rid the world of nuclear weapons by 2000.

With such efforts, Gorbachev hopes to regain the international prestige the Soviet Union lost when, under his three predecessors, it invaded Afghanistan in 1979, orchestrated a crackdown of martial law in Poland in 1981, walked out of U.S.-Soviet arms negotiations in 1983 and slipped into a spiritual and economic slump.

Some western diplomats here think that Gorbachev hopes to use the congress to draw the world's communists and socialists into a coalition of support for his peace proposals, including his call for a worldwide nuclear test ban.

"He will likely draw a scorecard between his own international efforts and those of the U.S.," said one senior Asian diplomat, "and then see who wants to sign on."

Gorbachev has gone to great pains to reach beyond his own party faithful in building an international audience for the congress, including unprecedented worldwide televising of the main proceedings. According to diplomatic sources here, the Soviet leadership has made special approaches to wary allies, such as Cuba's Fidel Castro and the Palestine Liberation Organization's Yasser Arafat, to come to the congress. For the first time, 16 delegations of western socialist parties, including those of some U.S. allies, have been invited as observers. But China, Moscow's biggest rival in the world communist movement, will stay away.

Soviet literary and cultural figures, too, are holding out hopes that the congress will bring a shift of the political winds in their favor. Several Soviet writers, publishers, artists and reform-minded economists have said that they will look to Gorbachev and other speakers to signal a loosening of the state's rigid hold on some business enterprises and on various cultural forums.

Seizing on the new leadership's call for more public criticism and openness, writers and publishers have been testing the official commitment to a cultural revolution. Several publishers have rushed works into print that gathered dust after Leonid Brezhnev brought an end to the Khrushchev thaw in the mid-1960s. One such work -- "Sad Detective," a short story by Victor Astafev -- was published last month in the literary journal October and greeted warmly by Moscow intellectual circles.

However, pending a green light at the party congress, movement in the cultural sphere has been minimal. In recent weeks, Moscow theater directors have used the official press to refloat a two-year-old proposal to take several local theaters out of state control. But, one well-known playwright said in an interview, "We're waiting until after the party congress to see what to do."

With an audience of party loyalists and a carefully scripted agenda, Gorbachev is assured a large measure of success at the congress. Approval of the party program, the new five-year plan and replacement of at least half the Central Committee members is assured, Soviet and western sources agreed.

The velocity of the winds of change motivating the Soviet leadership will be measurable in Gorbachev's and Ryzhkov's ability to lay out a convincing policy of economic reform in their policy speeches, according to western analysts here. That would involve filling in some of the gaps and clarifying the contradictions in the economic policy they have defined so far, the analysts feel.

These gaps and contradictions include:

The contradiction between the projected high levels of investment in retooling, computerizing and modernizing the country's industrial base and the levels of military spending that will be required to maintain competition in defense with the United States and NATO, and particularly with the Reagan administration's Strategic Defense Initiative.

The emerging difficulty in achieving the goal of a 150 percent increase in productivity by 2000 and the growth of income and availability of consumer goods over the same period, now projected to proceed at a snail's pace.

The apparent difficulty in achieving projected levels of output in the energy sector -- which accounts for two-thirds of Moscow's foreign currency earnings -- and the relatively low levels of investment planned in the energy industry.

If the Soviet leader cannot clear up these basic economic questions, said one western economist here, "we'll have to reassess the ability to achieve economic reform after the party congress."

Western diplomats have established two additional means of using the party congress to measure the extent to which the reform-minded leaders can operate with a free hand between now and the next party congress, in 1991.

The first is the composition of the new Central Committee elected at the end of the congress. An infusion of new members with secret police or military links may brake the pace of reform, in their view.

The second is the willingness of congress speakers to move the debates on economic reform, party morals and corruption from the columns of Soviet newspapers to center stage.

In the end, the 27th party congress, like its predecessors, will likely bear more directly on the lives of the 6.6 percent of the 277 million Soviet citizens who are party members than on the country as a whole.

If Gorbachev can demonstrate success at wresting change within the party, one Soviet official predicted, "change in Soviet society, the economy and across the country will come later."

If the proceedings follow the pattern set in recent congresses held in the Soviet Union's constituent republics, Gorbachev, Ligachev and others will spend nine days upbraiding their party unfaithful for corruption, bribery and moral decline.

Last month, Gorbachev retreated to a dacha near Moscow to write his keynote speech, a close aide said. When Gorbachev emerged, a journalist from L'Humanite asked if the Soviet Union was preparing to launch a "new revolution."

"Certainly not," Gorbachev answered.