The Italian senate narrowly voted down a controversian abortion bill today in a surprise move that could interrupt the increasing cooperation here between Italy's ruling Christian Democrats and its powerful Communist Party.

The defeat, by a vote of 156 to 154, of what would have been one of the Western Europe's most liberal abortion laws seems likely to bring this delicate issue back to center stage in Roman Catholic Italy 4 1/2 months after. The bill's passage in the larger Chamber of Deputies.

Today's negative vote, which means that six months must pass before another abortion bill can be presented to Parliament, is likely to recharge the nationwide abortion movement begun two years ago by the small Radical Party.

The pro-abortion vote in January was the Parliament's second major break from the traditional influence of the Vatican. It approved divorce bill in December 1970. The divorce law was reconfirmed in 1974 by a hotly debated popular referendum in which Roman Catholic groups were defeated by the strenous efforts of a Radical-Socialist-Communist coalition.

A thin majority favoring abortion had appeared to assure Senate passage of a bill that would guarantee abortion on demand to women during the first three months of pregnancy.

Jubilant Christian Democrats said tonight that the defection in the secret vote of seven senators from the pro-abortion camp probably reflected sincere "problems of conscience" and should not be given undue political significance.

But some supporters of the bill - which was backed in parliament by all of Italy's lay parties, from the civil-rights-oriented Radicals to the Communists and Socialists to the right-of-center Liberals - are convinced that the upset represented an attempt at interference with the slow process of rapprochement between Christian Democrats and Communists that has chracterized the Italian political picture for the last 10 months.

Since August, Italy has been governed by a minority Cristian Democratic Cabinet that depends on tacit Communist support in Parliament to survive. The two parties are involved in high-level talks that could lead to an unprecedented agreement on a joint program.

The touchiness of the abortion issue - right-to-life groups have filled stadiums in Rome and Millan for ralies of opponents of the bill - brought down one government in March 1976.

A similar outcome is not expected now, but the probable full-fledged battle on the issue could polarize the country as divorce did in 1974.

A Communist Party communique urged that the abortion question not be allowed to interfere with the inter-party negotiations in progress. The statement said, however, that a referendum is probably unavoidable.

Two years ago the Radical Party mobilized enough popular support to collect the 500,000 signatures needed to schedule an abortion referendum. Originally set for May 1976, the referendum was postoponed - first because of last year's early national elections and again this year because Parliament was working on a compromise bill. It will now be automatically scheduled for June 1978 and can be avoided only if a successful parliamentary compromise is reached.

According to some statistics, women here annually undergo more than a million illegal abortions every year. There are about the same number of live births. This situation has caused a bitter struggle, primarily by Radicals and feminists, against Fascist-period legislation providing prison terms for abortion and other crimes "against the integrity of the race."