As President Tito lies critically ill in a Ljubljana hospital, his successors have firmly rejected first signs of renewed Soviet pressures as a "slanderous campaign" designed to undermine Yugoslavia's independence.

A sharp exchange of polemics between Yugoslavia and the Soviet Bloc during the last week could provide a foretaste of some of the problems in store for the new Yugoslav leadership after Tito's death.

At first, most of the accusations were traded between Yugoslav and Soviet Bloc news media. But Friday a Yugoslav Foreign Ministry spokesman took the unusual step of warning of "a new campaign," apparently masterminded by the Kremlin, to split Yugoslavia from its friends within the nonaligned movement.

The immediate reason for Belgrade's concern are articles that appeared in the Vietnamese press accusing Yugoslav Foreign Minister Josip Vrhovec and other unnamed Yugoslav leaders of joining in a Sino-American propaganda campaign over Afghanistan. Alone among communist countries in Europe, Yugoslavia has bitterly attacked the Soviet invasion of Afghanistan and attempted to drum up support within the nonaligned movement for a condemnation of the Soviet action.

The Vietnamese articles were reprinted in the Soviet press, including the Communist Party newspaper Pravda. Other Soviet Bloc countries, particularly Bulgaria and Czechoslovakia, havce also carried critical press comment on Yugoslav foreign policy.

What may concern Yugoslav leaders even more, however, is the possibiltiy that the Kremlin might seek to present itself as the guarantor of Tito's "socialist achievements" after his death. Officials here noted that "certain" Yugoslav leaders are being singled out for Soviet criticism -- the implication being that they, not others, are departing from Tito's course.

While Tito, 87, was the first communist leader to break away from the Soviet Bloc, he always retained a direct personal and emotional link with the 1917 Bolshevik revolution and Leninist ideas. He fought on the side of the Bolsheviks during the Russian civil war and later returned to Moscow to work for the Comintern, the Soviet-dominated world organization of Communist parties.

Like Finland's veteran President Urho Kekkonen, Tito knew the Soviet Union intimately and was trusted by the Soviet leadership. When faced with Soviet demands, he knew when to be conciliatory and when to stand firm. Without his experience, the new Yugoslav leaders may find it more difficult to strike the correct balance.

The evidence so far suggests that the Yugoslavs are determined to stand their ground. In a recent private conversation, a senior Communist Party official here described the Soviet Bloc press attacks as "predictable," adding that they appeared designed to test the new leadership at a crucial time.

The Foreign Ministry spokesman, Mirko Kalezic, compared the publicity given to the Vietnamese allegations in the Soviet press with Soviet attacks on Tito in 1948 after Yugoslavia's expulsion from the world communist movement. He accused the Vietnamese newspapers of "intrigues" and "ideological warfare."

"Obviously this could represent some kind of new phrase in an anti-Yugoslav campaign," he said."It is very difficult not to form the impression that this amounts to the application of pressure on Yugoslavia's independent and nonaligned policies."

Few analysts, either here or in the West, believe that Yugoslavia is in imminent danger of a Soviet invasion on the Afghan model after Tito's death. But most accept that relations between Belgrade and Moscow could undergo new crises in the next few months, and the change in the government will be a crucial test both of the permanence of Tito's achievement and the strength of the new Yugoslav leaders.