Nobody here spends more time worrying about the physical and ideological threat of the Red Brigades than Italian Communists Party leaders.

With the Italian state already humiliated by the terrorists it is generally agreed that the next most telling blow they could deal to the country's institutions would be to kidnap Communist leader Enrico Berlinguer.

The Communist chief shares with Christian Democratic Party leader Aldo Moro the main responsibility for bringing about the "historic compromise" between Catholics and Communists that is the basis for the governmental agreement which Moro was on his way to ratify in parliament the day he was kidnaped.

To the Red Bridgades, the "history compromise" is a historic betrayal of the Communist Party's aims and traditions. It is increasingly clear that the party's rightward drift is largely responsible for unleashing the extreme leftist violence that was kept in check as long as there was a hope that the Communists would one day lead the revolution.

The violent groups of the extreme left have long since dropped any illusions on that score. Last year, 2,128 urban guerrilla incidents were counted, almost exclusively by groups claiming to be leftist.

Official Communist Party statistics show that, as the "historic compromise" has been moving closer to reality, the incidents have multiplied. The Communists counted 913 incidents from January through March, including 162 fire-bombings or explosive blasts against political parties' offices, many of them Communist.

In evident recognition that the party is now as vulneerable to physical attack as other groups, the Communists recently strengthened security measures at their headquarters in the Via delle Botteghe Oscure (Street of the Dark Shops).

Television cameras now record every approach to the building. There is a special garage for Berlinguer to get into his custom made armored car.

The communists fear they are infiltrated by the brigades. Interior Ministry sources said the brigades flag, a five-pointed yellow star on a red field, was found in the display case of the Communist bookstore next door to the party headquarters. The case could only have been opened with a key.

One of the rare police infiltrators into the brigadess testified at the current trail in Turin of Brigades leaders that their chief, Renato Curcio, told him in 1974 that a number of his followers still belonged to legal parties, especially the Communists.

In fact, the brigades probably did not have to go out of their way to infiltrate the party. They could simply ask party members disaffected by Berlinguer's policy of compromise to stay inside the party organization.

The extreme leftists are not the only ones who question the "historic compromse." Many thoughtful moderates are asking how democratic opposition can be channeled when the two dominant parties join hands to govern. Since the agreement was ratified by parliament March 16, only 10 deputies remain in the leftist opposition of the 630-seat lower house, the Chamber of Deputies.

But the Communists admit their obsession with what happened in Chile, where the left was democratically elected but the refusal of an electorally strong right to accept the moral ligitimacy of the leftist victory led to a military putsch. Berlinguer has often justified alliance with the Christian Democrats as a guarantee against the same thing happening in Italy.

The Italian Communist Party, showing unprecedented self-confidence for a major Communist Party, seems to have concluded that it can no longer occupy all of the terrain of the political left. The violence and effectiveness of the Red Brigades campaign against the historic compromise also seems to have led the Communists, out of a need to drive a wedge between the terrorists and the nonviolent extreme left, to admit implicitly that the latter has a right to exist in a democracy. Until fairly recently, Communist polemics lumped all of the extreme left as being "objectively" in league with the fascists. Communists aimed at "creating a desert to their left" in which "a myriad of small anti-democratic groups" sprang up, said Franco Ferraroti, head of the sociology faculty of Rome University.

The party vehemently denies that leftist terrorists are in any sense its children - despite such facts as the recent arrest for terrorism in Naples of the daughter of Amanuele Macaluso, a Central Committeeman close to Berlinguer. However, the party is agonizingly reappraising its moral responsibility for the climate in which terrorism arose.

After World War II, the Communists preserved a clandestine organization that grew out of the armed partisans bands that fought the Germans. They were led by Pietro Secchia, a hardline leftist who was party boss Palmiro Togliatti's deputy responsible for internal organization.

Togliatti ultimately purged Secchia, but not until a tradition of double-think arose to bedevil the party's protestations of sincere attachment to democracy.

"Until 1953-1954." said Giovanni Cervetti, the current member of the nine-man party secretariat in charge of internal organization, "certain comrades believed that the party's democratic line was only a temporary and tactical choise which would later be replaced by a revolutionary line."

Secchia was a close friend of the successful leftist publisher Giangiacomo Feltrinelli, who died in 1972 after an explosion, apparently while trying to blow up an electric pylon near Milan.

Feltrinelli toured Latin America and returned to Italy full of the armed-struggle doctines of Argentina's Montoneros, Uruguay's Tupamaros and, above all, Cuba's Ernesto Che Guevara. The publisher made speeches about the need to arm against an impending rightist coup (which was attempted in 1970, but literally fizzled out because of heavy rains on D-day).

Feltrinelli used his money to reestablish Secchia's network of armed proletarian groups. After the publisher died, the Red Brigades, who were just starting up, inherited much of Feltrinelli's network.

In a speech to the Central Committee Monday, Paolo Bufalini, another member of the party secretariat, called for self-criticism because the party tolerated such things as occupations of universities and schools, vandalism and violence against professors "as if they were forms of democratic struggle."

He said the party had ignored the growth of terrorism, extremism and violence on the left in its obsession with combating those evils from the far right.

The official party rejection of much of its revolutionary mythology seems bound to turn off even more of the romantic revolutionary fringe on the Italian left.

Bufalini condemned what he described as the acceptance of Latin American revolutionary examples without recognizing that they had to do with completely different situations from Italy. For the first time in an official party speech, he condemned the "romantic and simplistic myth" of "guevarism."

Only the week before, a visit to the antechamber of party president Luigi Longo revealed the portraits of Guevara and of Cuban President Fidel Castro side-by-side in places of honor.

When the Guenara protrait was mentioned to Antonello Trombadori, a particularly outspoken Communist parliamentary deputy and wartime anti-German resistance hero, he said:

"It is like the saints of the Catholic church who have been taken out of the official calendar but whose images are still hanging . . . the romantic Guevarist illusion has done a lot of harm to the world revolutionary movement and it has not yet been properly criticized . . . I would have given Guevara a medal for heroism and simultaneously condemned him to death for indiscipline."