Which word refers to a fervent supporter of a cause, group, person, or idea?

The term that denotes a fervent supporter of a cause, group, person, or idea is "partisan." This word encapsulates the essence of strong allegiance and often implies a commitment to specific political parties, ideologies, or movements. A partisan typically engages actively in promoting their chosen cause, aligning with the notion of unwavering support and advocacy.

While "patron" refers to someone who supports or champions a person, group, or institution often through financial contributions, it doesn’t carry the same connotation of active support for a cause. "Advocate" also embodies support but usually underscores a more formal stance in speaking or acting on behalf of a cause. Lastly, "follower" generally indicates someone who subscribes to or is influenced by a person or idea but lacks the passionate commitment that defines a partisan. Thus, the specificity of "partisan" in reflecting an ardent supporter makes it the most appropriate choice.