If you could get the Red Army into northern Italy in your alternate World War 2 timeline, the PCI should win the elections there. But the Americans, British, and the Catholic Church should be able to split off a rump Southern Italian kingdom allied with the capitalists.
Its worth keeping in mind that outside of two states in India, Communist parties have never won elections at the national or regional level. This is pretty much, like, everywhere. Communist governments have always shot their way in power or been installed by the Red Army. There have been some instances, including Italy itself, of really watered down Communist Parties participating in governing coalitions.
In neighboring Yugoslavia you got a Communist regime without the assistance of the Red Army, though elections weren't that important in installing it. But Yugoslavia is much less important than Italy is, whether strategically, economically, or culturally. The Yugoslav Communists also broke with Moscow much earlier than the Italian Communists did. I don't see the US government being willing to "Titoize"Italy, unless you have a timeline where the US is just not a factor on the European continent power World War 2. Even then, the British might be able to manage things on their own. Tito also didn't have to deal with the Catholic Church, on its home ground, in consolidating his regime.