MOSCOW, JUNE 17 -- Democratic Platform, a growing radical-reformist wing within the Soviet Communist Party, threatened today to form an independent party if the 28th Party Congress next month fails to make fundamental policy changes.

The group has wide support in party ranks, especially in urban areas, but will be represented by only about 125 delegates out of 4,700 at the July 2 national congress, the party's highest governing body. Delegates to a Democratic Platform convention here this weekend said their representatives to the congress would walk out if the party does not resolve to end its political control of the KGB, internal security police and the army, a move that could bring on the first factional split in the monolithic ruling party in nearly seven decades.

Platform members also passed a resolution calling on the congress to "renounce communism as the party's main goal, change simultaneously the party's name and proclaim its goal to be the building of a democratic society on the principles of freedom, justice and solidarity."

One Democratic Platform supporter, historian and legislator Yuri Afanasyev, described the coming congress as the party's last, and probably futile, attempt to restore its public standing. For their part, most platform members interviewed expressed pessimism that the congress would prove a catalyst for fundamental change, saying they were convinced that Soviet president and party leader Mikhail Gorbachev was not yet prepared to sever his links with the deeply conservative party old guard.

Platform member Alexander Tsipko declared: "A split, or even thousands of people just turning in their party cards, will solve very little. What most of us are really hoping and pushing for is to remake this party as a normal, parliamentary party that supports the creation of wealth, individual freedoms and social democracy. Any remnants of the old ideology and structures are unacceptable."

While Gorbachev has formally endorsed the emergence of a multi-party political system and new parties are already forming, the next elections here are scheduled for four years from now. Thus, for the moment, the Communist Party, with its watchdog cells in every workplace, army unit and public organization, is still the dominant political force in the country. It still controls vast resources and, especially in the provinces, more power than elected government officials.

According to party officials and foreign diplomats, Gorbachev's political strategy within the party has been taking shape over recent weeks. They say that while his ultimate goal is to transfer nearly all political power from the party to the government and to restore the party's public prestige before it goes the way of collapsed Communist organizations throughout Eastern Europe, he is not yet ready to give up the party's top spot for fear of losing control of it to a more conservative figure.

Party sources said that two key figures in the party's ruling Politburo, Foreign Minister Eduard Shevardnadze and Alexander Yakovlev, Gorbachev's closest ally on the powerful panel, favored splitting with party conservatives and creating a social democratic party but that they got no further support among the leadership. There are deep differences within the Politburo over essential questions of policy and philosophy, and Shevardnadze, a leading reform advocate, has carried on a long public feud with the Politburo's leading conservative, Yegor Ligachev.

Editors of the Communist Party journal, Kommunist, say that Gorbachev's strategy has been to soothe the conservatives but slowly to nudge the party toward many of the positions held by Democratic Platform. Subtly, Gorbachev and his advisers are changing the langauge and ideology of the party, and the party's ideology chief, Vadim Medvedev, even spoke approvingly of "private property" -- long anathema to party ideologues -- in a round-table discussion published last week in the party newspaper, Pravda.

Gorbachev and the radical-reformists are also concerned about the rise of a new conservative-dominated Communist Party in the Russian republic, heretofore the only one of the Soviet Union's 15 republics without a separate party organization of its own. At first, Gorbachev opposed creation of such a party, fearing that neo-Stalinists based in Leningrad would be among its key leaders. Since then, however, he has sought to mold it to his use and seems to have succeeded insofar as the Russian party appears likely to name as its chairman a moderate figure from Lipesk, Yuri Manayenkov.

Democratic Platform leader Vyacheslav Shostokovsky, rector of the Higher Communist Party School, said that a congress of the Russian party this week would be a kind of dress rehearsal for the Soviet party congress in July. At that national party gathering, Gorbachev is expected to dissolve the Politburo and create a new form of party leadership with a chairman, two deputies and a presidium of about 35 members, including representatives from all 15 republics.

Some current Politburo members, including Vitali Vorotnikov, Lev Zaikov and Nikolai Slyunkov are likely to lose their places in the leadership, but most observers expect Ligachev to remain at least for now, if only as a gesture to the conservative wing of the party.