Personally I like the definition that the historian Robert O. Paxton uses. Now, he’s a liberal, but he does have good insight into fascism and he doesn’t fall into that trap of deciding that communists and fascists must be the same thing. His definition isn’t materialist, but it’s a good start.
To paraphrase, his definition is “a suppression of the left among popular sentiment.” By left he means things like socialists, labor organizations, communists, etc. Fascism is a situation where a country has found its theater of democracy has failed and the capitalists need anything at all to keep themselves in power, even if it means cannibalizing another sector of capitalists. The fascists are the ideological contingent of this, who put forward a policy of class collaboration between working class and capitalist, instead of what socialists propose, which is working class dominance in the economy. Fascists exalt nationality or race because that extends through class sentiments. It brushes aside concerns like internal economic contradictions. I once had a comrade say something like “Fascism is capitalists hitting the emergency button until their hand starts bleeding.”
Communists using a vanguard party is to defend their own interests against capitalists or outside invaders. The praise of the CPSU in Stalin’s era was precisely because it acted as a development and protection tool for the working class. It did its job and people were wary of any return to the previous Tsarist or liberal governments. Women began going to school, women were given the vote for the first time. Pogroms ceased. In less than one lifetime of the CPSU administrating the country, people went from poor farmers to living in apartments with plumbing, heating, and clean medical care. That’s why there was such praise of the party, because they actually did things people liked, and they didn’t want anything to threaten them.
Also, what does it matter if there’s one party or two? The working class have a singular, uniting interest to overthrow capitalism. Why are multiple parties needed? Anything the working class needs to negotiate for can be handled within a socialist, democratic structure, not two or three competing structures against one another. Take a look at Cuba, which has one party, but doesn’t use their party to endorse candidates. Everyone’s officially an independent in the National Assembly.