"支持北部联邦者" can be translated as "supporters of the Northern Federation" or "those who supported the Northern Federation," depending on the context and desired nuance.
If referring to a specific historical or political group, "supporters of the Northern Federation" would be more precise. If emphasizing the individuals who actively supported it, "those who supported the Northern Federation" could also be appropriate.
In a more general or figurative sense, you might also use "advocates for the Northern Federation" or "proponents of the Northern Federation."