THE ITALIAN election returns show a startling departure from the most stable--not to say rigid--voting pattern in Western Europe. For 30 years, until last weekend, the Christian Democrats' share of the national vote hardly moved from one election to the next; in the last 20 years, over five elections, the variation had been less than 1 percentage point. This time, with very little warning in the polls or anywhere else, it dropped more than 5 points to 32.9 percent of the vote. Even more interesting, the Italian Communist party--the Christian Democrats' old enemy and the country's second- largest party--got none of those lost votes. The Communists' share also dropped, by half a point to 29.9 percent. The winners were the fragments.

One immediate question in this country is whether the Christian Democrats were injured by the prospect of American cruise missiles based in Sicily. No one knows. Yet if nuclear missiles had been a major issue, presumably the Communists, opposing them, would have done less badly. The results seem instead to suggest that a good many voters are fed up with both the big parties and in particular with the Christian Democrats, who have had the upper hand continuously since Alcide de Gasperi became prime minister in late 1945. The party is suffering in all the usual ways from too long a tenure. It has come to stand for complacency, passivity and recurrent scandal. To take the long view, the end of the Christian Democrats' phenomenally durable domination of Italian politics is probably going to be good for the country--ultimately.

The short run is another matter. The returns don't seem to promise much in the way of firm and vigorous government for the present. There will probably be a lot of activity among the five parties that picked up strength, from the neofascists on the right to the Socialists on the left. The likliest prospect seems to be a series of volatile coalitions and much frantic maneuvering in preparation for another election.

European democracy responds to the troubles of the European economy. Beginning with the British election of 1979 that brought Margaret Thatcher to power, every major country in Western Europe except Italy has changed course. In each, with that same exception, the governing party of the mid- 1970s has been thrown out and the opposition thrown in. That has not happened in Italy only because the principal opposition is the Communists. But this election makes it clear that a lot of Italian voters have begun looking for an alternative.