Italy's main political parties have wound up their electoral campaigns by concetrating an enormous proportion of their fire on a small party that is not expected to get more than 5 percent of the vote in national election Sunday and Monday.

Their target is the Radical Party led by Marco Pannella, 49, an extraordinary political showman who latches onto almost every issue, no matter how far out, that could elicit a few more votes.

Pannella seems to have identified the underlying issue of Italian politics that no one esle wants to talk about: the failure of the great ideological parties (Christian Democrats, Communists and Socialists) to solve Italy's problems.

The Radicals threw a major scare into the major political headquarters last year by forcing a national referendum on repeal of the sizable public financial support of political parties.

Some 46 percent of the electorate voted for repeal of a law that was viewed as a major reform of Italian political life when it eas passed in 1974.

The message of disenchantment with all the orthodox parties has not been lost on them, although they do not seem to know what to do about it.

Thanks to the Radicals, the basic issue of instituting legal divorce was finally settled in a referendum. They played an instrumental role in the passage of laws giving wives equal property rights and legalizing abortion, but they also esposue a great-number of causes that seem irrelevant to most Italians, such as legalizing marijuana, homosexual civil rights and abandoning nuclear power in a country that has no natural fuel resources of its own.

Pannella is described by envious opponents as a genius at attracting media attention by use of devices such as smoking marijuana at press conferences to provoke his arrest and leading hunger strikes at the Vatican to protest the plight of the world's starving children.

The politics of general protest and rejection are not new to Italy. That is what the Red Brigades' terrorism also is about. What is new is a pragmatic win-some, lose-some approach to issues that Italians used to consider American or British.

The big issue in the elections ostensibly is whether the Communists will be brought into the Cabinet by the Christian Democrats, but the general expectation is that the voting will not change much. Leonardo Sciascia, southern Italy's leading writer and a Radical parliamentary candidate, was expressing the general view when he recently told an interview, "the Communists will lose a little, the Christian Democrats will gain some. The Socialists will stay the same."

In the last national elections, in 1976, the Christian Democrats polled 38.7 percent, the Communists 34.4 percent and the Socialists 9.6 percent. It was a high-water mark for the Communists who had been steadily creeping up on the ruling Christian Democrats in every election since World War II. The Radicals got 1.1 percent of that vote.

The Communists admit they are worried that the Radicals may attract a large portion of the floating protest vote that went to the Communists for the first time in 1976. Communist strategists concede they have had only limited success in binding those voters to their party because most of them are middle class, and thus much harder to organize than workers in factories and labor unions.

The orthodox parties all attack Pannellas as a demagogue who shamelessly appeals to both the far left and the far right for votes.

The party the Radicals will probably hurt most is the Socialist, who have been desperately trying to reverse their gradual decline into oblivion.

The Socialists had encouraged the Radicals in many of their campaigns and had worked with them, thinking they could absorb them. They apparently were startled to find them serious rivals for the voters who are disenchanted with both the Communists and Christian Democrats.

Many Italians now say they are voting Radical because they are fed up with the "historic compromise" approach of the two main parties, which seek coalition formulas to govern together.

"Because of my experience with the Communists, I could not choose any party but the Radicals," said writer Sciascia, who was a Communist city councillor in Palermo, Sicily. "It is a party that wants to break the historic compromise, that wants to break this pact . . . The Radicals won't go into the government. They will perform an opposition role. The problem of Italy is that in the past few years there has been no opposition.

"The rules of the democratic game must be respected. There must be a party that governs and a party that acts as a watchdog. It everyone governs, in the end, no one governs."

It was a major coup for Pannella when he got Sciasci, 57, to head the Radical Party slate for both the national and European parliaments, along with another leading leftist intellectual, Maria Antonietta Macciocchi, 56. She is a former Communist parliamentary deputy from Naples who forced the party to hold a formal trial to expel her after she refused to follow the party line.

A former Paris correspondent for the Italian Communist party newspaper Unita, she is the ex-wife of Unita's present correspondent in Washington.

She said in an interview that she was attracted to the Radicals because they have no program, no ideology, no party structure and no discipline.

Macciocchi said she saw no comparable group eleswhere in Western Europe but she hopes to be able to use the European parliament as a platform to stimulate similar opposition to the status quo in other countries.

With just four members in the Italian parliament - two men and two women, one a former nun - the Radicals already have acted as a third force against the dominant Christian Democrats and Communists, says Pannella. The elections appear sure to bring more such nonconformists into the legislature.

Said Pannella, speaking of Communist leader Enrico Berlinguer, "if the Radicals get more than 5 percent, Comrade Berlinguer will fall and be replaced."