National elections two years ahead of time appeared inevitable in Italy today as the ruling Christian Democrats rejected as unacceptable the Communist Party's conditions for continuing to support the government in parliament.

The formal decision to give up attempts to form a new Cabinet is not expected until the middle of the week, after Premier Giulio Andreotti returns here form a European summit meeting in Paris.

The Cabunet crisis began Jan. 31 when the Communists withdrew their support for Andreotti. Communist sources said that, regardless of any public rhetoric that may suggest the contrary, the party's leadership had made a basic decision that it wanted to reenter the opposition after a year of being in the majority without sharing in Cabinet responsibilities.

Communist leaer Enrico Berlinguer, who had led his party's repprochement with the Christian Democrats, a policy known as the "historic compromise," was meeting increasing resistance from the rank and file. At local party meetings in preparation for a party congress now scheduled for March 30, there were widespread complaints that the party was getting very little if anything in exchange for its support of the government.

To preserve his leadership position, party sources said, Berlinguer had to abandon party elements that favor evolution away from revolutionary traditions and adoption of conventional West European social democratic approach.

Commouist Party strategists say they expect to lose a few percentage points in the elections and that the increasingly restive Socialist Party will pick up a few points, while the Christian Demorcrats will also gain. Analysts for other parties agree there will be no fundamental changes.

In the last parliamentary elections, in 1976, the Communists got 34.5 percent of the vote, the Socialists 9.6 percent and the Christian Democrats 38.8 percent.

The Communists continued to make heavy gains in regional elections in 1977. But in a series of local elections after the kidnaping and murder by Red Brigades terrorists of Aldo Moro, Berlinguer's Christian Democratic partner in Bringing about the "histric compromise," the Communists started to lose ground dramatically. The voters apparently accepted the view that the Red Brigades were a historical outgrowth of the Italian Communist movement.

Under the leadership of Bettino Craxi, the Socialists saw the opportunity for a comeback. Craxi has made it clear that the local elections last year made him think he could double the Socialist vote, regaining he position the party held after World War II as the representative of 20 percent of the electorate and making the Socialists once again the political equals of the Communists.

So Craxi started a drumfire of anti-Communist polemics that contrasted sharply with the general mood among the other parties of reconciliation with the Communists. The Communists seem to have decided that they would rather accept small losses for themselves and small Socialist gains now than sait two years with the risk that Craxi could win his gamble.

The Socialist leader now indicates privately that he will be satisfied simply to reverse the historic trend of Communist gains and Socialist losses. The Communists had steadily increased their portion of the vote in every election to the lower house of parliament since 1953.

In opposition, a Connunist source said, the party will devote itself to demonstrating that Italy cannot be governed without the Communist Party. The demonstration will go on for however long it takes to persuade the other parties that Communist desires must be respected, the source said. He estimated that could take six to 1 months.In that time, he said, no law the Communists oppose would be passed, and it would be shown that Italy cannot be ruled against the will of the working class, a broad hint of the likelihood of widespread strikes.

The Communists seem to have lost control of the labor movement lately. Against the will of the big communist labor federation, there has been a series of strikes by small independent unions in key sectors. The new tactics will give the Communists an opportunity to recapture the leadership of worker dissatisfaction. The Communist source said his party would wage an unprecedentedly intensive electoral campaign based on two questions: why were the Communists kept out of the Cabinet now that everyone admits that the party is not made up of revolutionary bomb-throwers? And how can this country be governed without the Communists?

Socialist leader Craxi and been banking heavily on the first direct European-wide elections for the European Assembly, scheduled for June 10, to help his party. Socialist parties throughout Western Europe are expected to do very well in that vote, and Craxi was maneuvering to ride the coattails of the other Socialist parties in Europe to a larger share of the Italian vote. Polls in Italy have shown that the Craxi approach was wellfounded. Now it appears that Italian elections will come before voting for the European Assembly.

The apparent purpose of all the recent attempts to form a new Cabinet with unusual and unlikely political combinations was for everyone to try escape the blame for calling early elections. The Socialists, who took the blame for the calling of the 1976 elections ahead of time, were heavily punished by the voters.

This time, the Socialists went out of their way to escape any rtesponsibility by offering for the first time in three years to join a Cabinet with the Christian Democrats. It was the Socialists' original decision to cut themselves off from the Communists and join up with the Christian Democrats that atarted their slide toward oblivion.

The Socialists posed as a condition for joining a Cabinet now the naming as ministers of some of the independent leftists elected on the Connunist ticket in 1976, a solution to the crisis that the Connunists had said they would accept. That concession was apparently the Communist contribution to the elaborate efforts to escape responsibility for elections.

On Friday, however, Berlinguer posed additional conditions that he said he knew would not be accepted. They included extending "historic compromise" governments, including both Communists and Christian Democrats, to city and regional councils, negotiating a joint program for the new Cabinet and granting all parties supporting the government a veto right over each other's Cabinet members.

Yesterday, a meeting of the Christian Democratic leaders confirmed that Berlinguer's conditions were indeed unacceptable.