MOSCOW, JULY 9 -- The Soviet Communist Party congress, in a tactical victory for President Mikhail Gorbachev, voted today to expand the composition of the party's ruling Politburo to include representatives of all 15 Soviet republics.

The change in party rules adopted today will alter the nature of the leadership body that has been regarded as the seat of ultimate power in the Soviet Union over the past seven decades. The new structure reflects both the devolution of political authority from Moscow to the republics and the general shift in power from the party to elected legislatures.

Gorbachev, having failed in his primary goal of using the congress to renovate the Communist Party from top to bottom, now appears to be attempting to limit the damage by preventing party conservatives from gaining too strong a grip on the Politburo and other leadership bodies. The decision to give the regional party chiefs ex-officio seats on the Politburo will dilute, in effect, what otherwise would have been a hard-line majority.

The 28th Communist Party Congress also adopted a toughly worded resolution on military policy today, declaring that the Soviet Union is still under threat from the West and needs to strengthen its armed forces. The statement appeared to be an effort to mollify senior military officers who have accused Gorbachev and Foreign Minister Eduard Shevardnadze of making too many concessions to the West without gaining anything in return.

"The congress considers that so far there are no guarantees of the irreversibility of the positive changes {in international relations}, and the military threat to the Soviet Union continues," the resolution said, adding that it is therefore necessary to "strengthen and maintain" the defense potential of the country.

Despite the predominantly conservative composition of the congress, there has been strong pressure from delegations representing the country's vastly divergent republics to assert their autonomy and secure greater representation in the central party leadership. More than half the delegates at the congress represent the non-Russian republics and were naturally inclined to support Gorbachev's move to restructure the Politburo in their favor. The proposal was approved by a vote of 3,325 to 839.

The composition of the current Politburo, which is made up of 12 full members and seven candidate or non-voting members, is heavily tilted toward Moscow-based politicians. The only non-Russian members are Shevardnadze, who is a Georgian, and Vladimir Ivashko, president of the Ukraine. Former Latvian KGB chief, Boris Pugo, is a candidate member.

Most of its current members, including some of Gorbachev's closest allies, are expected to be replaced when the congress winds up later this week, but Gorbachev himself now seems certain of reelection as party general secretary despite sharp criticism of his leadership by conservative delegates. Gorbachev suggested that the membership of the new Politburo be 23 or 24 after inclusion of top officials of the party's policy-making Central Committee -- each responsible for a specific policy area -- and a deputy general secretary to be elected by the congress. The 250-member Central Committee, also to be elected this week, is expected to be dominated by conservatives.

The decentralization of the party reflects the political trend toward transforming the Soviet Union into a confederation of fully sovereign republics, each controlling its own resources. Gorbachev said today that the new party structure would allow the 15 Communist parties in the republics "to act independently and really influence the Central Committee and Politburo."

The radical-reformist party splinter group known as Democratic Platform is expected to announce its intention to set up a rival left-wing party at a congress of its own later this month. The group is supported by fewer than 100 of the 4,700 delegates here, but claims the backing of up to 40 percent of the 19 million rank-and-file party members across the country.

Democratic Platform is itself split, however, on the best tactics to pursue to achieve its declared purpose of building a parliamentary force akin to the Social Democratic parties of Western Europe. One of its leaders, Vyacheslav Shostakovsky, head of the Higher Party School in Moscow, said today that he expects between 20 and 30 percent of the group's supporters to remain within the party.

The leading radical-reformist on the outgoing Politburo, Alexander Yakovlev, met with Democratic Platform supporters over the weekend and appealed to them to stay and fight for their beliefs within the party. He was quoted by the unofficial Interfax news agency as saying that the party's ultra-conservative wing was "historically doomed" and that a recent burst of activity by hard-liners was more like "death throes than a political line or posiition."

Yakovlev is one of several Gorbachev associates who have indicated that they prefer to work through state institutions, including the newly created presidential advisory council, rather than the Politburo. He told the weekend meeting that he would not seek reelection to the Politburo or any other top party post.

Today, Yakovlev accused the conservatives of trying to smear him by circulating an inaccurate account of his meeting with Democratic platform. "The political struggle at this congress is acquiring disgusting forms. There is a coordinated campaign against me," he told the congress.