That's not how class works, or the state. Class is not merely a level of power, but a social relationship, to ownership of the means of production and distribution. In any given class society, this distributes society into two broad classes, with various subcategories. The state is a result of this class struggle, and represents the ruling class of society, based on which class owns the commanding heights of the economy.
In socialism, the commanding heights are socially owned. Administrators do not have unique relations of ownership, but instead occupy socially necessary roles for the continuation of advancing production and distribution. In such a unified system, competing political parties do not at all prevent corruption, they enable it and accelerate it. The idea that decentralized competition in the political sphere is made to support a collectivized system is a bourgeois worldview of politics that sees the mechanisms of capitalism as eternal, rather than specific.
Contrasting with capitalism, the competition in the political sphere is the competition of the ruling class, which does not seek collectivization but instead a bigger individual slice of the pie. Where socialist administrators seek further collectivization and unification, capitalist administrators represent this or that wing of the capitalist class, which all fights in unity to preserve private property, and against each other for a bigger slice of the pie.
Socialist political parties are organizations of the most advanced of the working class politically. The most active, the most trained, the most developed. This does not make them a distinct class, this is a bourgeois view of class. Instead, it makes them occupy the social role of "proletarian intellectual" to borrow from Gramsci, which isn't a statement on intelligence at scale, but the social role they play in determining and shaping the thought of the entire proletariat as a class.
How do you envision socialism, with multiple political parties, not devolving into factionalism and breaking-up of society in a manner that allows the bourgeoisie to resurge?