The most conscious and politically organized fraction of the working class, which exercises over it an effective task of political guidance and direction.
The class as a party in capitalist society
In the first meaning of the term, party means an independent political subject: the proletariat "constitutes itself as a class and, therefore, as a political party", in the words of the "Communist Manifesto", when its own struggles, independent of any faction of the bourgeoisie, turn it into an autonomous political subject within capitalist society. This is the fundamental moment in the process of the constitution of the proletariat as a class.
Party "in the broad historical sense"
The "party in the broad historical sense" is one more result of capitalism and of the nature of the proletariat, and for that very reason it appears permanently, continuously, throughout its history, without necessarily implying a formal organization. By "party" we then mean the set of networks and milieus that express positions expressing the interests of the workers as a universal class.
Since 1852, then, I have known nothing of ‘party’ in the sense implied in your letter. Whereas you are a poet, I am a critic and for me the experiences of 1849-52 were quite enough. The ‘League’, like the société des saisons in Paris and a hundred other societies, was simply an episode in the history of a party that is everywhere springing up naturally out of the soil of modern society. [...]
I have frankly stated my views, with which I trust you are largely in agreement. Moreover, I have tried to dispel the misunderstanding arising out of the impression that by ‘party’ I meant a ‘League’ that expired eight years ago, or an editorial board that was disbanded twelve years ago. By party, I meant the party in the broad historical sense.
Marx to Ferdinand Freiligrath, 29 February 1860
The networks of individuals and groups that sustain such conversations continue to exist even in times when class struggle and affirmation are at a low ebb, because within them appear individuals, communists, who "learn lessons" from previous struggles and take part in the critique of the ideology that the system oozes.
When the proletariat does not react, it is only the force through which capitalism reproduces itself, taking advantage of the general apathy of its historical enemy. The working class is then only an amorphous conglomerate of people who manage as best they can to survive in an unbearable society, reproducing the atmosphere of competition and enmity, the unhealthy spirit of capitalism. In a word, it does not act as a class with common interests against the capitalist vermin.
At other times, however, the proletariat acts united as a class bearing the only possible human future: communism. Then two types of society can confront each other, one reactionary and the other revolutionary. Between these two states of affairs and these very facts there are individuals who are revolutionaries regardless of the momentary state in which the class as a whole finds itself; only their number varies according to the social situation. They are revolutionaries because they are aware that their goal, and that of the class as a whole, is communism.
These revolutionary individuals tend to organize themselves by affinity of ideas, ideas that do not fall from the sky but come from a particular interpretation of the history of the class struggle. Lessons are learned from every confrontation between capital and the proletariat, and from them revolutionary theory is born and what makes it evolve.
Class party
The development of that "party in the broad historical sense" in the course of struggles comes to form a relatively broad and organized minority of the class. On its ability to centralize itself around a program and a structure, however minimal, will depend the possibility of exercising, to a growing extent, an effective task of political orientation and guidance for the workers as a whole.
Let us be clear: the party does not "lead" in the manner of a company manager or a football coach; it does not give instructions that the workers "must follow". The class party is that most conscious part of the class which leads because it provides orientation, because it shows the movement as a whole a direction to follow.
The emergence of the class party as a fraction capable of providing direction precedes, and is necessary for, the constitution of the class as the leading party of society. The emergence of the "class party" is therefore linked to the development of the revolutionary process.
In general, it can be expected to take shape around the communist groups that have arisen as part of the "party in the broad historical sense". Although, as the Paris Commune showed and the Russian Revolution confirmed in different ways, what characterizes the class party and what is useful for centralization is not so much the particularities of the groups that converge, but the capacity to recognize the ultimate needs and possibilities of the struggle in its own development.