"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."
Karl Marx. Link.
"Today the German proletariat no longer needs any official organization, either public or secret. The simple self-evident interconnection of like-minded class comrades suffices, without any rules, boards, resolutions or other tangible forms, to shake the whole German Empire to its foundations.
/.../
The international movement of the European and American proletariat has become so much strengthened that not merely its first narrow form — the secret League — but even its second, infinitely wider form — the open International Working Men’s Association — has become a fetter for it, and that the simple feeling of solidarity based on the understanding of the identity of class position suffices to create and to hold together one and the same great party of the proletariat among the workers of all countries and tongues"
Friedrich Engels. Link.
We can also ask Pannekoek:
"The old labour movement is organized in parties. The belief in parties is the main reason for the impotence of the working class; therefore we avoid forming a new party - not because we are too few, but because a party is an organization that aims to lead and control the working class./.../Essentially the party is a grouping according to views, conceptions; the classes are groupings according to economic interests. Class membership is determined by one's part in the process of production; party membership is the joining of persons who agree in their conceptions of the social problems."
...or Bordigas clear division in a "formal" and a "material/historical" party:
"Therefore the concept of class must not suggest to us a static image, but instead a dynamic one. When we detect a social tendency, or a movement oriented towards a given end, then we can recognise the existence of a class in the true sense of the word. But then the class party exists in a material if not yet in a formal way. A party lives when there is the existence of a doctrine and a method of action."
Link.
"This distinction is in Marx and Engels and they were right to deduce from it that, being through their work in the line of the historical party, they disdained to be members of any formal party.
/.../
Marx says: party in its historical meaning, in the historical sense, and formal, or ephemeral, party. In the first concept lies the continuity, and from it we have derived our characteristic thesis of the invariance of doctrine since its formulation by Marx; not as the invention of a genius, but as the discovery of a result of human evolution. But the two concepts are not metaphysically opposite, and it would be silly to express them by the poor doctrine : I turn my back on the formal party, as I go towards the historical one."
Link.