Despite recent election results and the prestige-boosting chairmanship of two international summits in Italy this month, clouds darken the political horizon of Italy's affable prime minister, Francesco Cossiga, head of the current three-party government.

Unwittingly, and certainly unwillingly, Cossiga, 51, has become involved in a political scandal that has already cut short the career of a top-ranking fellow Christian Democrat, Carlo Donat-Cattin, and has made the prime minister himself the target of unprecedented impeachment-type proceedings.

The scandal, involving charges that Cossiga revealed official secrets and helped protect Donat-Cattin's terrorist son from arrest, has been exploited to the full by the Italian Communists who, despite a strong law-and-order position, have often found themselves blamed for the political terrorism here.

Aides to the prime minister believe that with this month's elections now in the post, the "Donat-Cattin Affair" is not likely to damage the government or to threaten the political survival of a man acclaimed as a "new Christian Democratic face." Yet it could drag on for months, providing an object lesson that in today's Italy moral issues and politics have become almost totally confused.

In addition, the scandal has refueled an old argument about the origins of political violence here. Although there are no cases comparable to that of Donat-Cattin, several well-known Christian Democratic leaders have children who have chosen political extremism.

Ever since 1977, when a phone call claiming responsibility for a terrorist attack was traced to Donat-Cattin's Turin residence, there have been rumors that the former Cabinet minister's youngest son had fallen in with terrorists. t

Nevertheless, the case broke only in mid-May when it became known that a young terrorist arrested in Turin in April had told interrogators that assistance from the top had helped Marco Donat-Cattin, 28, elude arrest.

Reportedly named earlier by imprisoned Red Brigades leader Patrizio Peci as having participated in at least one political murder, Marco Donat-Cattin was said to have disappeared shortly before a warrant was issued for his arrest.

According to accused terrorist Roberto Sandalo, Marco's best friend, Sandalo had been contacted in April by the elder Donat-Cattin and asked to warn Marco that arrest was imminent. Even worse, Sandalo claimed Donat-Cattin had cited Prime Minister Cossiga as the source of that information.

News of the confession, apparently leaked by a Turin magistrate and a member of parliament from the small Radical Party, fell like a bombshell on the otherwise uneventful campaign for this month's local elections held around the country. The immediate result was a decision by the parliament's permanent commission of inquiry to investigate the case.

The 20-member commission heard testimony from Sandalo, from Donat-Cattin, who was then the party's vice president, and from Cossiga.

The former Cabinet minister insisted he had been unaware of his son's activities until an anonymous "tipoff" led him to seek a meeting with Cossiga. But sufficient doubts were raised in the press to lead the Christian Democrats to ask him to resign as party vice president.

The hostile members of the commission, mostly Communists and neofascists, wanted to know if Cossiga initiated the meeting with Donat-Cattin and whether he revealed state secrets and somehow abetted Marco's flight.

Both Donat-cattin and Cossiga, who admitted meeting and discussing the matter, denied that Cossiga initiated the meeting or that he suggested that the son flee.

A majority of the commission chose to attribute the best possible motives to Cossiga's reply that there were no "specific charges" and that Marco ought to contact police.

The commission voted 11 to 9 -- along strict party lines -- to wind up the inquiry in Cossiga's favor. But this simple majority was not sufficient, under parliamentary rules, to permanently close the case.

With elections approaching, the Communists, neofascists, radicals and far left called for a new investigation. The voters largely ignored their attacks. But on June 12, while Cossiga was playing host in Venice to the leaders of the Common Market, his opponents made their move.

With Communist support, 394 signatures were collected among the 952 members of parliament, far more than the 318 needed to reopen the case.

Parliament, meeting in joint session, next will have to vote whether to send Cossiga before the constitutional court.

The eventual outcome does not appear in doubt since there is a pro-Cossiga majority in parliament, but one government source said the prime minister might well find himself on the "hot seat" during July.

The main question is whether the Communists will be satisfied with having made their point that the terrorist "family album" includes Christian Democrats as well as leftists.

The elder Donat-Cattin has said that 41 other politicians have children who are extremists and there has been a spate of press reports about the far-leftist offspring of several top Christian Democrats and other centrist politicians.

The progrovernment outcome of the regional and local elections June 8 and 9 considerably cooled the issue and Cossiga had made it clear he would not bow to pressure and resign.

Cossiga, a former Sardinian law professor was interior minister during the 54-day abduction of former prime minister Aldo Moro. He spontaneously resigned the day after the Christian Democratic leader was found slain. He returned only when unexpectedly tapped for the prime ministership in August 1979.

This time, however, he said, "There is a time for resigning and a time when it is right to remain at one's post."