Forty years after Juan D. Peron rose to power, and nine years after his death, a sense of expectation has returned to the ranks of his old opponents in Argentina's Radical Party.

The prospect is so momentous and has been so long awaited that many Radicals are hesitant to believe it. For two generations, this centrist, middle-class party has sought to wean Argentines from the almost mythical appeal of Peron's populist movement. In six presidential elections, one short-lived government, and four intervals of military rule, it has always failed.

And yet, less than 90 days before a new presidential election, Argentines have begun to talk of a new populist "national movement"--and its leader is not a Peronist but a Radical. Raul Alfonsin, the party's nominee for president, has fared well in the opinion polls and won the enthusiastic backing of many business, media and youth leaders.

Argentina, say the Radicals, may at last end its Peronist tradition in an open national election. "This is a Latin country that likes strong leaders, and a new leader has appeared in Argentina," said a party organizer. "It is Alfonsin."

The comparison may well prove ephemeral. Alfonsin, his critics say, has yet to be examined critically by voters or to confront a candidate of the Peronists, who are still mired in internal elections.

Nor has the Radical leader proved that his social democrat-like politics can penetrate the Peronists' formidable base in the unions and working class. Despite his advantage in the polls, Alfonsin is still widely considered an underdog for the Oct. 30 election.

Still, the stocky, 56-year-old lawyer, at once a career politician and a relatively fresh figure in Argentina's well-worn civilian leadership, seemingly already has altered the old equations. For the first time since Peron won his first election in 1946, the Peronists' place as the leading party seriously has been questioned.

And after decades of political instability and slow economic decline, shaped by the forces of Peronism and the military, Argentina has the prospect of a new political leadership committed to dismantling the destructive interplay of those two powers.

"The forces that represented decadence, violation and death have been exposed for all to see," Alfonsin declared with characteristic fire when he accepted the nomination of the 92-year-old Radical Party this month. "Now there will be no more frauds, and the same journey will not be repeated."

If Alfonsin were to win the Oct. 30 election, he would nominally be the third non-Peronist civilian president to govern Argentina in the past 25 years. However, presidents Arturo Frondizi and Arturo Illia, a Radical, won power in elections from which the Peronists were banned. They led weak governments that were destabilized by Peronist opposition and ended by military coups.

"People see Alfonsin as more decisive. He comes forward with a strong image of authority," said Roberto Crotes Conde, a political scientist. "A lot of people think a strong president elected with a strong vote would have the strength finally to put an end to some of our old problems."

It is precisely Alfonsin's aggressiveness that seems to have led to his surge in support in the last several months. Like Peron before him, he has sought to create a broad movement beyond the bounds of his party, appealing to left-of-center students and intellectuals as well as the middle class and the political right.

A former congressmen who long represented a left-of-center wing of the Radicals against a centrist majority, Alfonsin surged in popularity during seven years of military rule by actively supporting human rights groups and attacking the government with a charismatic flair.

Alfonsin has followed on old Peronist tradition by defining himself in terms of his enemies--in this case, the antidemocratic sectors of the armed forces, the "financial speculators," and the labor movement's Peronist-dominated bureaucracy.

These attacks have plunged Alfonsin into bitter disputes with military and labor leaders. Radical Party leaders accuse the Army's intelligence agency of mounting a propaganda campaign against Alfonsin, and Peronists have charged him with leading a foreign-backed alliance whose sole aim is to stop their movement.

Alfonsin denies that his campaign is based on anti-Peronism, saying instead that the principal issue is the return to democracy. However, he remains vehement in attacking the Peronists' weaknesses.

"We have to put an end to the corporativism and authoritarian demagogery in the popular sector," he said in a recent interview. "Peronism has these characteristics."

In fact, Alfonsin's chief tactical success so far may have been the limitation of Argentina's election to an essentially two-party contest that has focused less on policy issues than on the style of the future government.

Although a dozen or more parties may compete, the Peronists and Radicals are now considered the only important forces. The other parties have been defined increasingly in terms of which side they will support in the electoral college.

Radical Party leaders say the Peronists and Radicals now differ little in policies for the battered economy or foreign policy. Instead, one said, "the issue will be who you want to implement those policies, and in what way."

On this issue, the Radicals can offer a long history of relatively strong support for civil liberties and human rights. Alfonsin's platform is laden with liberal proposals, ranging from the removal of state-of-siege provisions from the constitution to the creation of a habeas corpus rule and the removal of television stations from direct government control.

In addition, Alfonsin has proposed a number of measures to limit the power of the armed forces and prevent further coups or political repression. He would abolish the service commander in chief posts, limit military spending to 2 percent of the gross national product and end mandatory military service. Any future battles against terrorism would be assigned to a special new security force that would be required to report to human rights groups.

In contrast, Radical leaders hope to paint the Peronists as a movement with authoritarian tendencies that has been plagued by corruption, antidemocratic procedures, and domination by its labor union affiliates.

"We have suffered a crisis that is a crisis of democracy," Alfonsin said. "And we don't have to demonstrate that we are a party of democracy. Argentina has suffered from a moral crisis. The Radicals don't have to prove that over the years we have maintained the highest ethical standards. We will not lose our way in getting what we want in the narrow paths of demagoguery and authoritarianism."

Such statements are meant to evoke the specter of the last Peronist government, ended by a military coup in 1976. That three-year administration, headed by Peron's last wife, Isabel, after his death in 1974, was plagued by political violence from both right and left Peronist factions as well as by allegations of corruption.

The most important move by Alfonsin in a year-long campaign for the Radical nomination, said several advisers, was his denunciation last May of an alleged "pact" between leaders of the Army and right-wing Peronist labor leaders. The union leaders are formally integrated into the Peronist movement's leadership and wield often decisive influence. Alfonsin charged that they planned to accept military favoritism in the reorganization of the labor movement in exchange for blocking any future Peronist government's prosecution of the military's past human rights violations.

The allegation was vehemently denied by the military and the Peronists, and Alfonsin still has not made public any proof of the pact. But his ratings have risen from below the Peronists' last April to a 37-to-28 percent advantage in late July, according to Radical Party poll information.

"He hit two sectors--the military and the union bureacracy--that are not liked by anyone in the population, even the Peronists," said Enrique Vanoli, a top Radical official and director of Alfonsin's media efforts. "People are hoping for a leader who will call things by their name."

Alfonsin's advantage has been increased in recent weeks by the Peronist struggles to settle on a presidential nominee. While two principal candidates, with former Senate president Italo Luder in the lead, have battled for control of delegates to a national convention, union leaders have sought to broker the choice. Isabel Peron, meanwhile, has remained silent in Madrid amid speculation that she may exercise personal power to name--or veto--the party candidate.

The Peronists still hold a commanding edge over the Radicals in party affiliations. Of 6 million Argentines registered with parties--about 30 percent of the electorate--about half are Peronists, while only about 1.4 million are Radicals.

"If all those working families who have always been Peronist simply go and pull the lever for Peron, it's all over," said Cortes Conde. "The people will go on being Peronists of Peron, but they could vote for Alfonsin because the party is not giving them the answers," said Luis Caeiro, a Radical Party leader. "That is what we have to hope for."