First of all, as I have often noted, the 1948 election was not at all close: The Christian Democrats got 48.5 percent of the vote for the Chamber of Deputies, the Popular Democratic Front 31.0%.
https://en.wikipedia.org/wiki/Italian_general_election,_1948 Transforming 31 percent into a majority isn't all that easy! Yes, the US helped the Christian Democrats, but it is hard to imagine a scenario in which they wouldn't have done so, unless the *world* situation (not just that in Italy) were drastically different from what it was in OTL. And in that event we might have a totally different election (the Communists might not have been excluded from the government, the Nenni Socialists might not have split with Saragat's social democrats, etc.)
But beyond that: Suppose that the Popular Democratic Front (FDP) did somehow win a majority. Would that mean a "Communist Italy"? Not necessarily. The FDP was after all an alliance of two parties, the PCI and Nenni's PSI. True, Nenni was generally sympathetic to the PCI and to the Soviet Union until the crushing of the Hungarian Revolution in 1956. But he was by no means a stooge of the PCI. "Part of the problem rested in the differing conceptions that communists and socialists brought to the Popular Front. The communists conceived it as something more than a temporary electoral alliance to unite the resources of the Marxist parties in a bid to unseat De Gasperi's government. The Nenni socialists saw things quite differently. To them the Popular Front was ephemeral, purely an electoral expedient, not an agent for long-term political socialization, and certainly not a new political party altogether..." Robert Ventresca, *From Fascism to Democracy: Culture and Politics in the Italian Election of 1948,* p. 173.
https://books.google.com/books?id=OKA6i7SenW0C&pg=PA173 In eastern Europe, the presence of the Red Army could assure that the Communists dominated the nominally "Popular Front" governments of the late 1940's--the Socialists were subordinated, and eventually forced to "merge" with the Communists to form "Workers'" parties that were really the Communist Party under a new name. But there was no Red Army in Italy. Of course the Communists might use control of the police ministry to force the non-Communists into submission, as in Czechoslovakia. But what if Nenni, for example, insisted that that Ministry go to a Socialist? In short, one should not automatically assume that an FDP-ruled Italy (which as I noted is something very hard to get in the first place) will be an eastern-European style "Communist regime."