ROME, JUNE 15 -- The Italian Communist Party, the largest Communist party in the western world, has suffered a significant electoral defeat in weekend parliamentary elections, in which the only clear-cut winner appears to be former prime minister Bettino Craxi's Socialist Party.

With 90 percent of ballots from the two days of voting counted, the Communists, Italy's second largest party, plunged to 27.1 percent of the vote, down from 29.9 in the last elections in 1983. The returns put the Communists almost seven points below their national electoral peak in 1976, confounding some predictions that the Communists could replace the Christian Democrats as Italy's largest party.

The Socialists were headed for their best showing since 1958 with 14.2 percent, up from 11.4 percent in 1983.

The Christian Democrats, which have led or participated in each of Italy's 46 governments since World War II, showed only a fractional increase, to 33.6 percent.

But the election of a new 315-member Senate and 630-member Chamber of Deputies yesterday and today, the nation's 10th legislative election since the postwar republic began in 1948, did little to alter the real balance of forces within the nation's noncommunist majority parties, and analysts and politicians immediately predicted another period of political instability before a stable government can be formed.

"From our point of view the vote was a good political result and I'm not surprised," Craxi said tonight. "We have reached a historic level of success, . . . the result of a long labor.

"But I hope we can get out of the political confusion in which we still find ourselves," he said. "It is clear nothing today can make one hope that things are better than they were yesterday.

"The situation remains very complex and confused," said Craxi, who, until his five-party coalition government fell apart with internal rancor this spring, had given Italy a rare 3 1/2 years of government continuity.

While Communist Party chief Alessandro Natta declined to comment directly on the results "until we have time to analyze them," other Communist officials were not so reticent.

With early official returns confirming previous grim computer projections, Giorgio Napolitano, one of Natta's senior lieutenants, declared the party's showing "a failure."

Napolitano and other senior officials said the party would have to spend a long time analyzing the vote and rethinking its policies as a result of the showing.

Some of the Communist vote, according to analysts here, seemed to have been lost to a new ecology-oriented Green Party that made an auspicious political debut this weekend, winning, according to partial returns, up to 3 percent of the vote. Other Communists, analysts said, may have shifted their vote to the more radical Democratic Proletarian Party, which made slight gains, or to the more moderate Socialists.

Most political observers here agreed that the formation of a stable government will ultimately depend on again forming the five-party coalition government that Craxi so successfuly headed until March.

Reconstructing such a coalition, however, is expected to prove difficult. Craxi was brought down because he would not accede to demands of the Christian Democratic Party that leadership of the coalition be rotated.

Craxi, as head of the second-largest party in the coalition, was picked to head the coalition in 1983 in part because none of the other parties trusted, or respected, the Christian Democrats.

Until the beginning of this decade, the Christian Democrats had led all of Italy's postwar governments. Since 1980, they have sought to dominate every coalition into which they were forced because of diminishing political clout.

The question that Italy's politicians must now face again is who is to lead the next coalition? So pressing was this question during the rather listless political campaign that preceded this weekend's election that no national issues were discussed.

The conviction here of analysts such as Alberto Ronchey, editorialist for Milan's Corriere della Sera newspaper, is that with the election over and the power balances between the parties reestablished, there will be new "political maneuvering and arguing" before any stable government is possible.

The argument is expected to be very much between the Christian Democrats and the Socialists, who both showed gains in the vote. The former coalition's junior partners -- Republicans, Liberals, and Social Democrats -- all lost votes tonight and thus are expected to have even less influence in the discussions than they had in Craxi's government.