The spic-and-span Communist headquarters of Western Europe's richest Communist Party, in both votes and money, would make an American politician drool. But the illusion that here is a Communist Party with "a human face" vanishes the instant that party operatives reveal their doctrine on questions of world politics.

Indeed, the decision of the Carter administration - though backed by Italy's democratic parties - to issue its manifesto of non-involvement in the political "processes" of Western allies haroring Communist parties may have come with undue haste and exaggerated emphasis. Party leaders here make preposterous claims out of it. ("Carter is looking at Italian reality in a new way," one told us.)

More important, the party's real view of the U.S.-Soviet struggle is chillingly anti-American, yet the Carter "non-involvement" policy issued in April now makes it difficult for the United States to advertise that fact without violating its own edict.

Sergio Segre, a leading Communist specialist in foreign affairs and member of the ruling central committee, found himself unable to say whether the United States or the Soviet Union gives the "higher expression" to human rights. Calling it a "senseless comparison," Segre told us in a rebuke to Carter that "to be avoided at all costs" was any U.S.-Soviet "confrontation" on the human-rights question - for example, pitting the condition of American blacks against the plight of Soviet dissidents.

How about imperialism, a much-favored slander against the United States? We asked Segre to consider not just Soviet military control of Eastern Europe but also current Soviet activities in southern Africa and elsewhere, compared with last year's refusal by the United States to get involved in Angola.

The response was quick and confident: "Even your own officials like [U.N. Ambassador Andrew] Young and President Carter himself have said that Communist troops in Angola created stability, and that is not imperialism."

As for Eastern Europe, Segre told us, Moscow has troops there as part of the Warsaw Pact, just as the United States has troops in Western Europe as part of NATO. But, he was asked, is not the purpose of Soviet troops in East Germany, Poland, Hungary and Czechoslovakia to guarantee Moscow's military control and economic domination of those countries?

"There is a question about what function Soviet troops have in Eastern Europe," Segre said. He added the party's routine disclaimer on the Soviets' 1968 occupation of Prague.

Embroidering that curious perception of imperialism, Ugo Pecchioli, an articulate, 52-year-old executive committee member with growing influence in the party's foreign-policy apparatus, came close to equating U.S. membership in NATO with "imperialism."

"Certainly the American presence has conditioned the political development of the countries of Western Europe," he told us. "[Consider] the prevalence of American interests, American armaments - NATO uses American arms, not European. And we have learned about the CIA operations from hearings in your Congress" (a reference to undercover U.S. help for democratic parties in past Italian elections).

And Soviet imperialism? "There is no element of Soviet imperialism in Eastern Europe," he said. "Imperialism totally suffocates the life of a country." He repeated Segre's line about Soviet-Cuban intervention in Angola. "Even the U.S. has admitted in the last few weeks that Cuban troops in Angola brought in element of stability and democratization."

Given these convictions, to swallow the Communist Party's new doctrine on NATO - that, should they ever attain power here, the Communists would continue Italy's NATO membership but press hard for the "obsolescence" of both pacts - requires a staggering act of faith.

If much else is unclear about Italy's second largest party, this fact emerged from our discussions: The new doctrine on NATO, which was widely advertised in last summer's election campaign, seems to rest on a foundation of hot air.

Considering the party's contorted view of the U.S.-Soviet struggle, its pledge to tolerate continued membership in NATO is not principled. Rather, it is merely a tactic to appear more acceptable to the Western-oriented middle class in its appeal for votes - a tactic that, in other areas, appears to have damaged the party's working-class base, triggered a potentially serious inner-party debate over future strategy and, at least, for the moment slowed its drive toward power.