"主张分裂者" can be translated into English as "secessionists" or "advocates of separation".
"Secessionists" refers to those who advocate for or support the separation of a region or group from a larger entity, often with political or territorial implications.
"Advocates of separation" is a more straightforward translation that directly conveys the idea of supporting or promoting separation.
Both expressions are appropriate depending on the context and the level of formality required.