A mass demonstration of Argentina's Peronist party was threatening to collapse here one night last week when Herminio Iglesias, the party's blunt-spoken gubernatorial candidate in Buenos Aires Province, stepped before the microphone.

More than 100,000 persons had gathered in and around a soccer stadium to cheer the campaign of the populist Peronists for national elections Sunday. But a series of speeches had not roused the crowd, and the party's top official, Lorenzo Miguel, had been stopped from speaking by a barrage of whistles, sticks and stones from rival party factions.

Once again, the mass movement that has dominated Argentine politics for two generations was foundering on its controversial leadership and long-standing internal rivalries. But Iglesias, the lean, graying machine boss from a working-class suburb, knew how to unite the crowd.

"We have to return to our doctrine," he shouted hoarsely from a podium at one corner of the vast stadium. "Because this Oct. 30, it is not Herminio Iglesias who is going to win, it is not presidential candidate Italo Luder . . . but Juan Domingo Peron who is going to win again."

A roar erupted from the two-tiered grandstand. The rally ended well. And this Sunday, most political experts here predict, the appeal of Peron, Argentina's dominant postwar political leader, will once again shape the outcome of a national election--this time more than nine years after his death.

If the old mystique is enough, Luder, 66, a moderate, relatively unglamorous constitutional lawyer, will be elected to a six-year term. Peron's Justicialist Party will take power for the third time since l946, and eight years after its last administration under Peron's widow Isabel ended in political violence, economic failure, and a military coup.

The Peronists say they have changed since then. The party's extremist sectors on the left and right have been disarmed and at least temporarily demobilized, and the movement's traditionally authoritarian internal power structure has been moderated by democratic procedures. Isabel Peron, the erratic and unpopular last president, has apparently retired from politics, and Peronist economists say they will be more flexible in applying their statist, redistributive economic policies.

And yet, the Peronists freely concede, their movement has not yet consolidated its new structure of leadership or clearly defined its approach to policy making. Luder, an experienced politician, has been unable to establish clear control over either the party's powerful labor wing or its traditionalist right-wing bosses.

Thus, even as that institutional struggle proceeds, what is holding the party together and driving the campaign is little more than the mystique of Peron--and his vague, sometimes contradictory ideological legacy.

For most Peronist activists, this intangible appeal is more than enough. "Peronist doctrine is not defined by ideology but by feelings," explained Carlos Funes, a former youth leader close to Peron. "We feel first, then we rationalize."

The immediate question for Peronist leaders, however, is whether this traditional home-grown attraction will be enough to win another national election. For the first time in their history, the Peronists face a severe political challenge from a democratic opposition, led by Raul Alfonsin, the charismatic candidate of the center-left Radical Party.

Alfonsin, who drew more than 600,000 supporters to his final campaign rally in Buenos Aires Wednesday, has clearly cut into the Peronists' working-class base and is considered capable of upsetting the party in Sunday's vote. Even if he does not, say analysts here, he has managed to hammer home the Peronists' long-range problem of leadership in a new government.

"The top Peronist leaders have said that no candidate will win the elections, but that instead Peron will win," Alfonsin reminded the vast crowd that filled Buenos Aires' wide Avenida 9 de Julio. "If that is true," he roared, "who is going to govern Argentina?"

That is a question that even Peronists find difficult to answer. "The movement is still searching for its leadership formula, and there are two alternatives," said Simon Bermudez, the head of a Peronist business association. "Either we will find that leadership, or we will disappear."

For now, the party's following seems to have concentrated around three poles, each with a leader of national stature and a distinct style and ideology. Luder, Miguel and Iglesias together have come to represent the legacy of Peron during the campaign. Each is vital to the party's vote, but their frictions have threatened both the campaign and any future government.

For many Peronists, Luder represents Peronism's move toward institutionalizing itself as an internally democratic and even relatively traditional social democratic party. A former senate president and interim president, he is unquestioned as a political professional and his mild rhetoric may have reassured many voters troubled by the perceived excesses of the last Peronist government.

Luder, however, is viewed by most analysts as dependent for support on Miguel, the conservative leader of the powerful Peronist labor-union bureaucracy. That labor superstructure forms the most powerful element within the Peronist party.

For the Peronists, the labor unions are crucial to delivering the massive blue-collar vote they depend on to win elections. But once in government, critics here say, Luder may be unable to prevent Miguel and other conservative labor leaders from dictating unacceptable economic policies. Miguel, moreover, has been charged with corruption, antidemocratic practices, and even the backing of right-wing death squads.

Neither Miguel nor Luder has drawn widespread popular enthusiasm in the election campaign. Instead, it is Iglesias who has seemed to create his own base of support in the crucial electoral zone of Buenos Aires Province--and Iglesias is the most controversial Peronist politician of all.

A former metalworker with knife and bullet scars to show for his colorful past, Iglesias has built a classic machine on patronage and populism--and, according to public charges, numbers games and racketeering--in a area that one Argentine analyst compared to a Mafia-dominated section of New Jersey.

Iglesias has waged a relentless campaign of economic populism, appeals to nationalism and invective against opponents, defining critics as representatives of "foreign interests." He does not explicitly deny his reputed right-wing military contacts and ominously hints that Peronists cannot accept defeat in the elections.

"There is no alternative," he said in a interview, reclining in a chair under color portraits of Juan, Evita, and Isabel Peron. "Peronism already won."