For the first time since World War II, the Italian Communist Party suffered a setback in national elections today, losing three percentage points in the vote and 22 seats in the lower house of parliament.

The ruling Christian Democrats, instead of making expected gains, suffered a slight decline in popular vote and lost two seats.

A host of smaller parties were major winners, according to computer projections. Most were located between the Christian Democrats and the Communists on the political spectrum. The Radicals, who ridiculed the major parties, more than tripled their vote.

The prospect was for more short-lived cabinets like the outgoing government of Premier Giulio Andreotti. It fell Jan. 31 because of the Christian Democrats' refusal to bring in Communist ministers as the price for continued Communist support in parliament.

It was the 40th government since the end of the fascist dictatorship 36 years ago.

A highly respected computer projection tonight showed the Communists as the biggest losers in the 630-member lower house, slipping from 228 seats to 206 with 31.2 percent of the vote.

This was a 10 percent loss of seats, while the popular vote loss was 3.2 percentage points to the last election in 1976. Italy's system of proportional representation magnifies the raw-vote losses of larger parties.

The radicals were the biggest gainers, going from 4 seats to 13.

The Christian Democrats, projected to lose two seats, would wind up with 260. Their share of the vote would drop .6 percent to 36.1. The Socialist Party is likely to gain two, for a total of 59. The Liberals were expected double their representation, going 5 to 10 seats. The Social Democrats were seen jumping from 15 to 20; the neo-fascists were seen gaining two seats, for a total of 36. The Republicans held 15 seats, one more than last time.

These results greatly strengthen the patchwork of center parties-Socialists, Social Democrats, Republicans and Liberals-giving them a total of 104 seats, 13 more than before.

Christian Democrats need to find at least 56 additional seats to put together a ruling coalition. This may not prove impossible since they have ruled with junior coalition partners since 1953.

The results gave a paradoxical double impression both of stability and of disaffection. They were in line with the comment of a north Italian businessman shortly before the vote: "People don't know who to vote for, only who to vote against. They're not choosing who they want but who they don't want."

The Communists' losses were particularly high in southern Italy - 7 percent in Naples and 10 percent in Palermo, Sicily, according to still incomplete returns.

At least initially, the results should strengthen the hard-line elements inside the Communist Party who argued that cooperating with the Christian Democrats without getting major concessions from them was a serious tactical error.

Those elements have served notice that they would accept a new pact with the ruling party only on Communist terms and that Communists should be ready to use strong methods, such as waves of strikes. But the hard-liners undoubtedly will meet resistance from those who say the party must project a responsible image to gain a formal share of power.

The Communists, who have always gained in successive elections, took 34.4 percent of the vote in 1976-only 4 percentage points behind the Christian Democratic Party and a spectacular gain of 7 percent over their previous total.

That great leap forward was attributed to a combination of disgust with Christian Democratic rule and a feeling that the Communists of Italy had demostrated their attachment to democracy and could contribute to reforming a system ruled by one party for more than 30 years.

But the Communists failed to force serious reforms and today's results demonstrated that they were unable to hold on to the middle-class floating votes that had turned to them.

This time, the protest vote went to swell the parliamentary ranks of the Radicals and the more traditional Liberals and Social Democrats. But the Socialists, whose leader Bettino Craxi, maneuvered vigorously to restore his flagging party as a third force, barely managed to raise their sowing from 9.6 percent in 1976 to 9.9 percent.

Craxi struck a strong anti-communist note, pledging to the party that this would net big results. His failure to deliver could endanger his leadership of the party. His challengers are likely to argue that the results showed that his brand of violent anti-communism no longer attracts Italian voters.

The outcome seems likely to have a strong influence on the evolution of the "historic compromise" between the Communists and the Christian Democrats. Under the leadership of Enrico Berlinguer, the Communists pressed for years for the political respectability of being accepted by the Christian Democrats as partners in government.

When Christian Democratic Party leader Aldo Moro finally convinced enough of his followers to give the Communists a chance, he was kidnaped by the left-wing terrorist Red Brigades while en route to the parliamentary session that was to consecrate the accord on March 16, 1978.

Moro's "execution" 55 days later was explicitly aimed at killing the historic compromise, according to Red Brigage communiques. While the terrorist action momentarily strengthened the alliance, the Communist rank and file became increasingly restive over what was seen as a meager return for the party's support of the Andreotti government.

Berlinguer was pressured into an increasingly hard line until the break came in January. Yet, both he and Andreotti have said they are interested in pursuing the historic compromise. It is generally assumed that many of those who abandoned the Communists after voting for them in 1976 were upset by the party's close association with the Christian Democrats.

Another form of protest was probably at work in the number of persons voting-90 percent, compared to 93 percent in 1976. This was the lowest turnout under the Italian republic.

Italian voters consistently produce astonishingly high turnouts compared to other Western democracies. This is partly because there is a strong stigma attached to not voting. It can cost a promotion for a civil servant, and an ordinary citizen can suffer such consequences as delays in getting a passport-although the law forbids such reprisals.

Eligible to vote were 36 million Italians over 25 years old for the Senate, and 42 million Italians over 18 for the dominant lower house, the Chamber of Deputies. The vote for the Senate was slightly more conservative than for the Chamber.

The public opinion pools accurately predicted the Communist losses, but they misleadingly predicted that the Christian Democrats would gain several percentage points.

Contrary to the trends in 1976, which were national, there were strong regional differences this time.

Southern Italy's turning its back on the Communists was explained by most Italian observers as a reaction against Communist pledges to bring efficient government to Naples-which proved highly resistant to Communist attempts to turn it into an urban showcase like Bologna in the north. The Communists have ruled there for decades.

Another problem the Communists faced was the persistence of terrorism. Although the Communists constantly denounced these crimes, their strategists admit that they were hurt by the popular identification of the "reds" of the terror brigades with the "reds" of the Communist Party. CAPTION: Picture, Italy's Christian Democratic Party President Flaminio Piccoli, in Rome, discusses election returns. AP