Next Step : For the Gorbachev Doctrine, Now Comes the Hard Part : * At the G-7 summit, all eyes were on the Soviet leader. But tougher tests await him back home.

MOSCOW — President Mikhail S. Gorbachev, fresh from his encounter with leaders of the Group of Seven, faces a different inquisition this week from his own comrades in the Soviet Communist Party, who fear that he is indeed abandoning socialism for capitalism.

Their questions, including many from Marxist-Leninist die-hards, may be even sharper than those last week from the leaders at the annual summit of the major industrial democracies in London, where Gorbachev sought to get help for his crisis-ridden country by emphasizing his commitment to free-market economies.

“The West is pushing for bolder, faster, deeper reforms--and that is good,” Vladimir I. Shcherbakov, a first deputy prime minister, commented in London last week. “But Gorbachev is under heavy pressure, and not just pressure but a real political threat, too, from our conservatives who say, ‘No, no, no,’ and who want a return to what they regard as the true faith.”

The next showdown will come in the Kremlin on Thursday when the Communist Party’s policy-making Central Committee will consider a proposed new platform, a reformist program preparing the party to compete in a pluralist, parliamentary democracy and support a mixed, market-oriented economy.

The platform’s adoption would have fundamental importance for the Soviet Union since the Communist Party, although reduced to 16.1 million members after more than 3 million resignations in the past three years, remains the strongest political force in the country.

The platform recognizes the changes brought by perestroika , as Gorbachev’s political and economic reforms are known, and makes them the party’s new creed.

No longer will the party claim, as it did in the platform adopted in 1986, to be preparing for “the planned and all-round perfection of socialism, for Soviet society’s further advance to communism through the country’s accelerated socioeconomic development.”

Instead, the platform accepts that wealth is created by entrepreneurship, market forces and competition, not by central planning and state ownership, and that the party’s role is to argue the interests of the working class, particularly for social protection, and to defend public ownership where it is still appropriate.

But party conservatives, angered by what they see as Gorbachev’s abandonment of Marxist-Leninist orthodoxy, have been preparing for battle, vowing to defeat the platform--and impeach Gorbachev.

“We must decisively reject Gorbachev’s anti-people policy,” Nikolai Merzlikin, the Communist Party’s first secretary in Chita in eastern Siberia, said in an article last week in the newspaper Rabochaya Tribuna. “The Communist Party must refuse to give Gorbachev another vote of confidence. He has no right to call himself a Communist and make the party his hostage.”

Merzlikin and 10 other influential party leaders from Siberia have now been joined by conservatives from other areas in demanding the convocation of a special party congress to reverse the recent reforms and elect an orthodox general secretary to replace Gorbachev. Such a move could further split the badly fragmented party.

“Gorbachev is personally responsible for the sharp worsening of the working people’s life,” Merzlikin said. “Can we entrust him with the leadership of the party and the country? We cannot.”

While most of their questions will be directed at the continuing Soviet economic collapse--the gross national product has shrunk a further 10% so far this year, foreign trade is down 37% and retail prices are up 48%--Gorbachev’s quest for Western assistance is certain to draw criticism.

In the view of conservatives, Gorbachev has surrendered to imperialism, forsaking socialism to secure Western loans and credits, mortgaging the country’s considerable wealth to foreign investors and selling off public assets to the country’s own emerging capitalists, many of whom made their money on the black market.

“Call me a conservative,” Merzlikin said, “but nobody needs such democracy when you have nothing to eat and nothing to wear. When the policy of the party leadership breaks away from the interests of the workers and peasants, I am not going to keep silent.”

The conservatives want, most of all, to restore the party’s constitutional monopoly on political power and then work to re-establish the country’s Stalinist economy, which was based on virtually total government management of industry, agriculture and commerce.

“The creation of the Bolshevik Platform is the only way of preventing the catastrophe that the country is heading for,” Nina Andreyeva, the leader of one of the new ultraconservative groups, said at its founding conference earlier this month. “We must return to our fundamental principles and end this sellout to capitalism and world imperialism.”

Gorbachev has not only survived such challenges before but strengthened his position by besting the conservatives and rallying liberals and moderates behind him.

“These people might be able to kill Gorbachev politically through plots and conspiracies, by getting his own officials to knife him in the back, but they don’t have the votes to throw him out,” a Central Committee member who accompanied Gorbachev to London said. “They have tried this, and failed each time.

“But perestroika is more vulnerable. Economic reforms can be sabotaged, as we have seen, and the people can easily be turned against them because they are difficult to accomplish and life is now much harder. . . . So, Gorbachev is not fighting for his political life, but for the success of perestroika. “

The coming weeks are crucial for Gorbachev as he seeks to recover perestroika ‘s early momentum and use it to promote further reforms, and the Soviet political calendar is full:

* After the Central Committee meeting, Gorbachev next week will receive President Bush here in a long-delayed Soviet-American summit to sign a treaty reducing the superpowers’ strategic arsenals by nearly a third, burnishing the Soviet president’s reputation as a peacemaker.

Although the treaty is likely to draw conservative opposition, particularly within the politically powerful Red Army, it will free resources for the civilian economy.

* The Union Treaty, which will lay a new constitutional basis for the Soviet Union, is nearly ready, and its implementation will reverse the present flow outward from the center of political power. A new constitution and then national elections, for president and Parliament, should follow within a year.

The treaty’s signing by nine of the country’s constituent republics should also resolve much of the political and ethnic discord that has threatened to tear the country apart. Although six republics--the Baltic states of Estonia, Latvia and Lithuania and Armenia and Georgia in the Caucasus Mountains and Moldova on the Romanian border--have said they will not sign, virtually all are bidding for access to the Soviet Union’s “common economic space.”

* Key economic reforms should be undertaken in the next two months. The privatization of state-owned enterprises will start; the government will begin to break up longstanding monopolies to bring market forces into the economy through competition; supply and demand will govern prices of roughly 80% of goods, and the Finance Ministry and State Bank will introduce strict austerity measures, including sharp cuts in subsidies, to reduce the government’s budget deficit,

Plans will also be made for the conversion of large segments of the defense industry to civilian production, probably with Western investment, for making the Soviet ruble convertible, step by step, into freely traded currencies and for the liquidation of 15 more ministries, after 20 in the past year, that run different Soviet industries.

“There is a feeling that we have a chance to get things right in the next few months,” Leonid I. Abalkin, a leading Soviet economist and former deputy prime minister, said in London last week. “I am not talking about getting out of the crisis quickly; I am talking about turning our situation around slowly. We have that chance.”

Gorbachev, freeing himself from the considerable pressure that conservatives exerted on him over the past year, built up important momentum in recent months.

He reached a truce with Boris N. Yeltsin, the immensely popular president of the Russian Federation, the country’s largest republic, and then worked out a series of agreements with other republican leaders on the Union Treaty.

He pushed the Soviet military into far-reaching agreements with the United States and its NATO allies on arms reduction, including the prospective treaty reducing strategic weapons.

And, in the process of preparing for the Group of Seven conference, he forced through fundamental decisions on transforming the Soviet economy.

“No one element is decisive, but all are crucial,” a Kremlin insider said last week. “The most important thing perhaps is psychological--restoring confidence in the process of reform, in Gorbachev’s leadership, in our ability to do this.”

Gorbachev, concluding his visit to London, made a similar observation. “People are impatient--they want to go to bed, get a good night’s sleep and wake up to see everything changed,” he commented in a television interview. “It won’t happen that way. . . .

“We are in for difficult times, really difficult. But other countries have gone through such things, and so will we. It will be hard, complex and painful. But we will get there, and I think that our people now realize this and are ready to do what needs to be done to bring about change.”