I see that ‘big tent’ or ‘catch-all’ parties are quite popular in modern politics. However, the autocrats of the past took the opposite view:
Before we can unite, and in order that we may unite, we must first of all draw firm and definite lines of demarcation. <...> we do not intend to make our publication a mere storehouse of various views. <...> A party becomes stronger by purging itself. (Lenin)
It is quite erroneous to believe that the strength of a movement must increase if it be combined with other movements of a similar kind. <...> Through the formation of a working coalition associations which are weak in themselves can never be made strong, whereas it can and does happen not infrequently that a strong association loses its strength by joining in a coalition with weaker ones. (Hitler)
Why were Lenin and Hitler opposed to the unification strategy that gives the majority to many contemporary European parties?
update: Why, for example, CDU/CSU and SPD in Germany don't split into separate parties that would argue with each other?