The Italian Communist Party, the largest Marxist party in the West and on the verge of national political power just four years ago, is foundering now, and returning to hardline radicalism in an apparent attempt to regain its diminishing support.

With 201 seats in the 630-member Chamber of Deputies, 31 percent of the popular vote in the last election and administrative control of almost all of Italy's major cities, the communists are still a significant political force here. But there is little doubt that the party and its leader, Enrico Berlinguer, have lost much of their appeal and power.

In the June elections, the Communist Party's share of the vote dropped for the second year in a row. The resolution of Italy's most recent central government crisis last month, in which a broad-based coalition was formed that excluded the communists from any role in running the country, is also seen as a defeat for Berlinguer's party.

But the most telling sign of the party's desperation, in the view of many observers here, is the ultramilitant stand it took during the recent month-long strike at the giant Fiat automobile plant in Turin.

Now the Italian Communist Party's return to a long-abandoned form of hardline opposition has cast doubt on the soundness of its policies as well as on the longterm democratic development of the party and its future role in Italian society.

The party's allout pro-union position in Turin contrasted sharply with its past policy of building alliances with business, religious and political groups and appeared to be the most extreme manifestation so far of a gradual return to radicalism that began about two years ago and increased sharply following election losses June.

As recently as four years ago the communists' outstretched hand to the middle class won the party widespread acceptance or at least tolerance and contributed to hefty electoral gains that brought it closer to power than at any time since 1947.

Today, it is not unusual to hear politically involved Italians describe the communists as "bankrupt," "finished," "washed up."

Many Italian observers see the party as floundering and wonder if it is being outmaneuvered by the smaller civili-rights oriented Socialist Party, which returned to the government last March for the first time in six years. There is speculation, that the once forward-looking communists could turn into a hardline workers' party like that in France, doomed to a role of permanent opposition.

In the aftermath of the Fiat strike many party insiders and sympathizers are disturbed that, despite strong party support for the strikers, the bitter confrontation ended with a settlement considered favorable for Fiat as well as with an unforeseen split in the labor movement.

Many non-communists were deeply disturbed by Berlinguer's decision to go to Turin during the strike and tell workers that should they decide to occupy the plant the party would provide moral and material support.

Fiat chief Gianni Agnelli, who in the past had given the impression he felt he could work with the Italian communists, told a television interviewer the day after the Oct. 17 settlement that he thought recent events in Turin "had strengthened the viewpoint of those who have little faith in the communists' capacity to coexist in a democratic society."

Berlinger's speech during the strike "was a colossal psychological error," said a socialist university professor from Florence who believes the party has lost its sence of direction. "How can they believe it is possible to back even the most extreme demands of the workers while at the same time demanding a fullfledged government role."

The general explanation for the party's turnaround is that 2 1/2 years of close cooperation with the national solidarity governments headed by Christian Democratic premier Giulio Andreotti between 1976 and 1978 had confused and troubled the party rank-and-file.

In 1979 the Communists broke with the Christian Democrats, citing a change of attitude by the ruling party after the terrorist murder of their chief political strategist, Aldo Moro. The Communist policy reversal was not sufficient, however, to avoid a 4-percentage-point loss in the 1979 national elections and new losses last June are thought to have convinced Berlinguer that he must tighten his party's relations with the masses.

The Italian Communist Party's change of direction does not appear to include its foreign policy positions. Over the last year the party has accentuated its already substantial differences with Moscow by a more open-minded attitude on the West's planned tactical nuclear missiles, by sharply criticizing the Soviet invasion of Afghanistan, and by outspoken support for the Polish workers.

Last spring the Italian Communists refused to attend a Soviet-sponsored conference of Communist parties in Paris and it has reestablished relations with the Chinese.

A notable development in recent months has been the increased criticism of Berlinguer by party members, if not in public party meetings then in closed-door encounters and social events at which outsiders are present.

But the wiry Sardinian's position within the party is not endangered. Even his critics admit that Berlinguer, a centrist, is the only leader capable of holding the party's right and left-wings together.