A letter written in captivity by Aldo Moro. Italy's kidnapped former premier, has presented the fragile, Communist - support Christian Democratic government with an agonizing dilemma: what to do if left-wing Red Brigade terrorists holding Moro suggest negotiations for his release.

The handwritten letter, in which Moro, president of the Christian Democratic Party described himself as a "policical prisoner exchange, has been judged authentic by Moro's associates and family.

Government officials appear convinced that the letter addressed to Interior Minister Francesco Cossiga, the head of the Italian police, was written by Moro under duress or the effects of drugs.

The Italian government has not yet commented on what position it would take if the Red Brigades were to seek negotiations. But a top-level meeting of Christian Democrats, including Premier Giulio Andreotti and Interior Minister Cossiga, helped draft an editorial for today's edition of the party newspaper. II Popolo, which says blackmail by the Red Brigades is unacceptable.

The long communique from the Red Brigades letter - which was left in litter baskets in the streets of Rome, Milan, Genoa and Turin Wednesday night - set no condition for the release. Moro was kidnapped in a ambush two weeks ago. His five police body-guards were killed in the attack.

More's rambling references in the final part of his five-page letter to examples of other prisoner exchanges led many politicians to believe that the terrorists may try to propose that Moro be exchange for imprisoned terrorists.

All of Italy's parties, including the powerful Communists, have denounced the release of the letter and its contents. The Communist Party daily Unita, in today's front-page editorial, says, "The democratic state cannot give in to terrorism."

A spokesman for the Vatican, which Moro mentioned in his letter as a possible mediator, said it "has never refused to carry out humanitarian actions."

The Italian government has not yet commented on what position it would take if the Red Brigades were to seek negotiations.

After two weeks of fruitless investigations - more than 360,000 people are said to have been stopped at road-blocks - some observers have speculated that Premier Andreotti might come under pressure from fellow Christian Democrats eager to save the life of their party's major political strategist.

Others say, however, that if the party were to agree to negotiate with the terrorists for Moro's release, its credibility would be irrevocably damaged.

Moro refers in his letter to the 1977 exchange of Chilean Communist Luis Corvalan and Soviet dissident Vladimir Bukovsky, and to West Germany's release of five jailed terrorists in 1975 in exchange for kidnapped politician Peter Lorenz.

He did not mention the case of Mario Sossi, the Genoa judge kidnapped by the Red Brigades in 1974 and released after Italian judicial authorities made a later unfulfilled pledge to release several jailed members of Genoa terrorist band.

Experts here say that after that experience, the Red Brigades gave up the use of hostages for this type of bargaining. These experts believe the main purpose of the letter was to discredit Moro, considered the principal architect of the current Communist-Christian Democrat truce.