The pale face of Isabel Peron stared out from blue posters on street corners. Her taped voice rang out from raspy radio rebroadcasts. Her loyal followers marched in the streets with her slogans painted across long white banners.

But the ex-president and widow of Juan D. Peron had not come home in triumph for today's opening of the Peronist party's national convention.

Nor was she in Rome to consult with the pope, as one sector of a virtual industry of Peron watchers and unofficial spokesmen had suggested. She was not even in nearby Paraguay, where she is reported to have a house awaiting her.

Instead, the last civilian president of Argentina and reluctant heiress of its mass populist movement apparently spent the day as she has spent the summer--secluded and silent on an exclusive beachfront in Fuengirola, Spain. Puzzled Peronist leaders did not know when or if Peron would return to Argentina. They were unable to determine either her wishes for the party or her personal political plans.

And so, somewhat reluctantly, the Peronist movement embarked on what could be the consolidation of a historic change. Without either the mystique or the authority of a Peron as its personal leader, the country's dominant party set out to approve a platform and candidates for next month's national elections.

It was a task that quickly proved difficult to manage. A court ruling last night blocked the seating of about one-third of the delegates, forcing Peronist leaders to delay their proceedings. Meanwhile, they continue to seek an end to bitter disputes between rival factions.

By this evening, it appeared that the legal difficulties and internal feuding might indefinitely delay the party's nomination of a presidential candidate. The result, party leaders said, would further damage the party's image and risk paralysis following turbulent and occasionally violent provincial conventions.

And yet, a majority of party leaders appeared to have unified behind the presidential candidacy of Italo Luder, 66, a former Senate president and constitutional lawyer. Luder, a relative moderate, has not been seriously challenged since a summit of party leaders endorsed him last month.

If Luder can be nominated without serious difficulties, Peronist activists said, a long-term shift in party leadership could be clearly established. Rather than depending on the dictates of a single leader, so long the case under Juan Peron, the movement would be shaped by compromise among its unwieldy collection of factions.

And in Isabel Peron's absence, the movement appears likely to be dominated by its powerful base in labor unions, whose leaders have already become brokers of the national convention.

"All of this process has been timed to keep Isabel from becoming mixed up in the action," said one prominent party leader, asking that he not be named. "The historical balance is changing. It's evident that the labor sector has shown great force in taking control of the party."

The shift in Peronist leadership--and accompanying dangers of dissolution--has been developing ever since the 1974 death of three-time national president Juan Peron. Plunged into the presidency by her husband's death, Isabel, a former cabaret dancer with a grammar-school education, lacked the flair of Peron's previous wife, Evita.

By the time her government ended in the military coup of March 1976, Isabel Peron was being ignored by much of her movement. As Peronism, united only by its vague nationalist ideology, split into factions, the labor-union bureaucracy created by Peron in the 1940s emerged as the strongest.

She spent five years under military arrest, then, in Spanish exile, she withdrew into a small circle of trusted friends. In the past year, she has refused to make any public statement. Many analysts believe she has little popularity among voters in Argentina.

But as the Peronist party has prepared for elections this year, many leaders--including those of the labor sector--have seemed unable to exorcise the movement's personalist myth. "It is the one factor that brings all Peronists together," said former congressman Juan Labake.

Thus, even as Peronist authorities have worked to rebuild their organization with largely democratic procedures, effusive partisans and their press in Buenos Aires have complicated those efforts by stressing Peron's role as a leader.

While Peron, 52, has steadfastly refused to speak or meet with Argentine politicians, her supporters in Argentina have portrayed her as a decisive figure likely to be confirmed as chief of the party or even its presidential candidate.

Even as Peron has relaxed on the Spanish coast--and Argentine newspapers have filled with pictures of her in bathing suits--likely and unlikely spokesmen have surfaced in Buenos Aires almost daily to "announce" her plans or wishes for the Peronist party. Invariably, the reports, usually centering on Peron's return, have been contradicted or denied by other "spokesmen."

Two months ago, Peron's bodyguard, an East European immigrant immediately labeled "the Croatian colonel," was said to be in South America. His rocky physique and dark glasses were featured on magazines and his political role in a return by Peron were seriously discussed for days. Peronist leaders denied he had ever left Madrid.

With Peron's refusal to act, the gap in the Peronist power structure has been largely filled by its labor wing. In recent weeks, the chief of the Peronist labor organizations, Lorenzo Miguel, and other union figures have dominated selection of key candidates and brokered the feuds among competing political currents.

Peronist leaders here still maintain that Peron will return in the coming days to endorse the movement's campaign for the Oct. 30 election. By most accounts, Peron is said to be awaiting the annulment by military authorities of a criminal conviction and other charges against her.

Government officials have been quoted as saying a pardon for Peron is being considered and could be granted by late next week, after the scheduled end of the Peronist convention.