Party tradition and history are important factors in relation to its identity. But they are not the only overdetermining factors. Leadership characteristics, possibility of new articulatory practices, international affiliations, alliances, political opportunity strutctures, etc are other overdetermining factors which can result in identity construction.
Hence, for example, one can ask: Is the Labour Party of today a continuation of the past, a shift, etc? Whatever the case, it is surely a construction of a plurality of discourses (which at times are contradictory). Similarly, there is no a priori rule which says that other parties cannot conduct their own articulatory practices.
I believe that this is how one should discuss matters relating to Political party identity - such as Mario De Marco's statement that the Nationalist Party today is to the left of the Labour Party.