For example many CPSU members were formally members of the social democratic party, almost all Eastern Bloc communist parties were mergers of the preexisting communist parties with social democratic parties, and the Korean Social Democratic Party is one of the few opposition parties in the DPRK
It was not voluntary.
Please explain
The man in your pfp used the power of violence to crush fascism and try to fold the sucdems into actual socialists. Didn’t work, sadly.