You will find below an excerpt where Nahuel Moreno provides a simple definition of the party, and the specific role played by Marxist parties. Despite this text’s brevity, we believe it helps to dispel many misconceptions and ground more important debates on political organization. It is excerpted from an informal interview Moreno gave a year before his death.
 
The Political Parties and the Working Class
(…) With the French Revolution new and never before seen institutions appear: the political parties.  The groundwork for these parties was laid by all the previous development of the bourgeoisie. There cannot be a struggle without a political leadership, and we can expand that definition to the whole history of humanity that is the history of class struggle, the struggle of the different class sectors with their respective leadership. And since the French Revolution we know that the best way to organize those leaderships is to form political parties. What is a party? It is a voluntary regrouping of individuals that leads or pretends to lead one class (or a particular class sector), and in many occasions, an alliance of classes and sectors, and it acquires permanent characteristics regarding its structure, methods of functioning, and program.
Let’s address now a very important theoretical problem. Marx argued in his time, that the working class would have a single political party. However, after he died, we saw that there were in the proletariat different sectors, with their corresponding parties. We cannot deny that those parties have a very solid social basis: for example the Social Democracy, the parties of the Second International, are based on the labor aristocracy.
As Trotskysts, we believe that the proletariat needs a revolutionary leadership at the international level, a leadership that is lacking today. This is why we need to build the party. It is the modern expression of a class’s, or class sector’s, leadership.
Q: Does that mean that the hypothesis of Marx “one working class, one party” revealed itself to be wrong?
It was correct in its epoch and during the following years, at the time of the emergence and formation of the modern proletariat. In the 19th Century, when many bourgeois parties already existed, Marxists affirmed that in the proletariat there was a strong centripetal tendency that will favor the existence of a single party for the working class, given that the working class is way more homogeneous that the exploiting class.
In the 20th Century the situation became more complicated, especially after the emergence of imperialism. We have different historical and structural formations, there are also immigrants [and oppressed sectors], there are new trades or professions. A new “middle class” strata has emerged, that for me is a sector of the working class because it lives off of the wage system. The different worker’s parties reflect different sectors of the working class, just as bourgeois parties represent different sectors of the bourgeoisie, even though the proletariat is less scattered and never had so many parties as the bourgeois.(…)
 
Translated from the original Spanish by Daniel Iglesias