The threatening black-and-white image in the giant-sized wall poster is that of a deadly nuclear missile in flight. The message is that those who want peace should cast their ballot for the Communists.

Elsewhere, two glossy pages of a popular woman's fashion magazine are taken over by the gaping mouth of a menacing tank cannon and warnings against unchecked rearmament. But the political directive at the bottom of the page is the same. Concerned by predictions that it will lose ground in national elections here on Sunday and Monday, the powerful Italian Communist Party, Italy's major opposition group and second-largest political party, has intensified its campaign offensive against the installment of intermediate-range nuclear missiles in Italy.

Unless an agreement is reached in the ongoing Geneva nuclear arms negotiations between the Soviet Union and the United States, the North Atlantic Treaty Organization will begin installing in December 108 Pershing II and 464 cruise missiles in Western Europe.

Paradoxically, in Comiso, where a U.S.-staffed air base is being prepared to host 112 cruise missiles, the Communists appear to have tempered their antimissile campaign. Sicilian political sources said this week that because the city's unemployed were eyeing jobs at the base with interest, the missiles were not figuring as prominently as had been expected in the local Communists' election rallies.

In contrast, last week in a major speech in Rome's Pincio Park, party leader Enrico Berlinguer told a mostly young crowd of several thousand that it was the "vanguard of a great international movement, the peace movement."

"A vote for the Communist Party is a safe vote for all those who are fighting against the missiles stationed in the East and in the West, for those who want a positive conclusion to the Geneva disarmament negotiations, for those who want a total freeze of nuclear arms until they have been outlawed completely," said the 61-year-old Sardinian. Berlinguer sharply criticized Italy's current government, led by the Christian Democratic Party, for what he termed "teacher's pet" behavior in following the "directives" of NATO and the U.S. government.

The slight, graying leader feels very strongly about the planned installation of cruise missiles in Comiso, and he has returned frequently to the issue in recent election speeches. Nevertheless, in past months, the party has failed to put its full weight behind its antimissile convictions.

To some observers, the Communist Party's antimissile stance seems to conflict with its acceptance of Italy's NATO membership. Now, however, the Communists have stepped up their antimissile campaign and, with the exception of the small, far-left Democratic Proletarian Party, are alone in using the missile question as an election theme.

Scheduled deployment of the nuclear missiles, which the Italian government first agreed to in December 1979, is supported by the dominant Christian Democrats, the three small centrist parties that have participated in most of Italy's recent government coalitions, and by the Socialists, who have held the Defense Ministry portfolio for the past three years.

When Socialist Party leader Bettino Craxi spoke at the Communist Party congress in March, he gave the impression that he might be softening his promissile position. By saying that the Geneva negotiations should continue until an agreement is reached--and neglecting to mention the timing of deployment-- Craxi led some observers to believe that the Socialists felt deployment should be postponed.

Socialist officials later denied that there was any change in their position. A Western diplomat said that although the Socialists were not always as vocal on the missile issue as the Christian Democrats, the Socialist Party was "firmly on board" in its support of the NATO position.

Although a strong peace movement with firm roots in both church and society, such as that in West Germany, the Netherlands and Britain, has failed to develop here so far, most noncommunist politicians seem to be cautiously avoiding the missile issue in their election speeches. In Comiso, the question of the base has been downgraded by both the Communists and spokesmen for the incumbent Socialist-Christian Democratic administration there, Sicilian political sources said.

A traditional Communist stronghold, Comiso is one of a small number of Italian cities where a municipal vote is also to be held Sunday and Monday. With more jobs in the offing as preparations are made for a new round of bids for base-related building and construction contracts, the missile issue has assumed a different dimension, the sources said. When expressed, Communist opposition has centered on the possible disruptive effects that a U.S. base could have on small-town Sicilian society.

In contrast, in their national campaign the Communists appear to be banking on the conviction that the issue deeply concerns young voters who otherwise might abstain from voting. In the last elections in 1979, the Communists--who consistently have been Italy's second-largest party in both voting power and parliamentary representation since 1948--won slightly more than 30 percent of the vote, a significant drop from their 1976 score of 34.5 percent. Although after 12 years as the party's leader, Berlinguer's position is still basically unchallenged, political observers here have speculated that sharp political losses for the Communists could undercut his standing significantly.

Several recent published and unpublished polls have indicated that this year the party could fall below 30 percent for the first time since 1972. A top party official said privately last month that he would consider the elections a success if the party managed to hold to its 1979 level.

A poll published Friday commissioned by the conservative Milan newspaper Il Giornale predicted that the Christian Democrats would repeat their 1979 election performance with 38 percent of the votes, while the Communists would show a slight drop, from 3l.8 to 30.8 percent. The Socialists showed a gain of nearly two percentage points, from 9.8 to 11.7 percent, The Associated Press reported.

Along with the missile issue, the Communists also have been hammering on what they see as an antilabor swing by the Christian Democrats and their coalition allies during a time of serious economic difficulty. The inflation rate is high, at 16 percent, and unemployment has increased to about 2.3 million. The staggering public deficit is estimated to be more than $50 billion, and in April industrial production fell by a record 14.8 percent.

In the late 1970s the Communists worked closely with the Christian Democrats in a pact of national solidarity, but this "historic compromise" was abandoned by the Communists in March in favor of a "democratic alternative" based on an alliance with the Socialists.

But this year's electoral strategy was sharply undercut by Craxi in late May when the Socialist leader ruled out as premature the idea of the "alternative." Last week Craxi proposed a formal pact with the Christian Democrats after the elections and was roundly criticized by the Communists for "selling out."

The dilemma for the Communists is that when they try to appeal to Italy's independent, non-Marxist, progressive voters, as they did successfully in 1976, they risk alienating their hard-core supporters. On the other hand, should they decide to forgo attempts to please the moderates, the Communists may be condemning themselves to the role of a permanent opposition.

According to Communist Party figures, in 1982 close to half of the party's 1.7 million members were industrial or farm workers, and this does not include the 18.6 percent who are pensioners. In contrast, the professionals, teachers and students who are heavily represented among the cadres and leadership make up only about 7 percent. The stalemate between traditionalists who want to preserve the party's strong sense of social and political identity and modernizers who favor increasing its social democratic characteristics has paralyzed party policy on issues that could cost the party at the polls.