I’d just like to interject for a moment. What you’re referring to as Leninism, is in fact, Marxism-Leninism, or as I’ve recently taken to calling it, Marxism plus Leninism. Leninism is not an ideology unto itself, but rather another free component of a fully functioning Marxist ideology made useful by the 『Manifesto』 , 『Capital』 and vital ideology components comprising a full ideology.
Many communists run a modified version of the Marxism ideology every day, without realizing it. Through a peculiar turn of events, the version of Marxism which is widely used today is often called Leninism, and many of its users are not aware that it is basically the Marxism ideology, developed by Marx and Engels.
There really is a Leninism, and these people are using it, but it is just a part of the ideology they use. Leninism is the methodiology: a set of working methods for achieving the goal of communism. The methodiology is an essential part of an ideology, but useless by itself; it can only function in the context of a complete ideology. Leninism is normally used in combination with the Marxism ideology: the whole ideology is basically Marxism with Leninism added, or Marxism-Leninism. All the so-called Leninism distributions are really distributions of Marxism-Leninism!
Good bit, lol!
Lenin’s advances on Marxism are largely in analysis of imperialism, and practical revolutionary tactics such as democratic centralism, as well as to combat the revisionism of the second international. Trotskyists often try to call themselves “Leninists” to differentiate themselves from Marxism-Leninism, or “Marxist and Leninist,” but their own theory diverges from Marx’s.
Lenin’s contributions are not diversions from Marxism, but additions and updates to follow the development of imperialism, which pushed the contradictions Marx thought would lead to revolution in Europe into the global south, and all the new questions that added.
For anyone that wants an intro to Marxism-Leninism, I made an intro ML reading list you can check out!