Executive summary: It is reasonable to conclude that during the play, there were no Herodians onstage. There is no reason to think that in real life, followers of Herod were visually distinctive. In addition, according to the text of GMark, the Herodians did not contribute to the action. The name is a distinction without a difference. Because Mark’s purpose was to preserve the performance (in which the Herodians did nothing), I believe he did not add that name to his polished text. He was not writinge a history of Judean sects. I believe that “Herodians” was added by an editor to Mk 3:6 and 12:13. Now for pure speculation: the editor’s motive. I speculate that the editor was responding to the query of someone who had read Josephus’s description of the three Judean sects (Jewish War 2.8.2). This reader of Josephus wanted to know “what did the Essenes and the Sadducees think of Jesus/the Jesus figure?” The editor now altered Mark’s text to answer that question. The editor disguised Judean Essenes as “Herodians.” I infer that the editor wanted to continue to use the positively charged name “Essenes” for an earlier group of Essenes. This scenario also explains why the term “Sadducees” appears suddenly in the text of GMark (Mk 12:18). During the performance, the audience had only seen “chief priests,” and had never heard the spoken word “Sadducees.” The editor, I propose, simply changed a reference to “chief priests” in Mark’s text to the equivalent term, “Sadducees.”
“Herodians” are mentioned in GMark at 3:6 and 12:13. Both times, the Herodians appear as adjuncts to Pharisees. The Herodians do not speak and there is no reason to think they are identified to the audience in the dialogue of the play. Why not? They do not do anything distinctive in those scenes, or later.
A principle of playwriting is “don’t burden the audience’s memory with unnecessary information.” Because the Herodians don’t do anything in the play, there’s no reason for Mark to have made them visually distinct from the Pharisees. Or for their name to be spoken. So for this reason—they are superfluous to the play—I think that they were added by an editor to Mk 3:6 and 12:13.
Why did the editor change Mark’s polished text? There has to be a good reason in the world of the editor.
In 1967, Constantin Daniel proposed (in French) that “Herodians” was a term for Essenes. (See a review by Willi Braun, “Were the New Testament Herodians Essenes? A Critique of an Hypothesis.” Revue De Qumrân 14, no. 1 (53) (1989): 75-88. https://www.jstor.org/stable/24609005?seq=1). I think that Daniel’s proposal is right, at least for the references in GMark. (I don’t know if Essenes were actually known as “Herodians” on the ground in Judea.) According to Josephus (Antiquities of the Jews 15.10.5), Herod the Great had favored the Essenes. It is reasonable to assume that the editor and readers of GMark were familiar with Josephus’s works and would have needed little help to link “Herodians” in GMark with the Essenes of Josephus.
Why would the editor have added “Herodians” to Mark’s polished text? The simplest explanation is that the editor was responding to the query of a reader who had read Josephus’s description of the three Judean sects (Jewish War 2.8.2) and wanted to know “what did the Essenes and the Sadducees think of Jesus/the Jesus figure?” The editor realized that Mark’s text didn’t mention those two sects by name. So the editor altered Mark’s text. The editor disguised Essenes as “Herodians.” He placed them in association with Pharisees on the question of healing on the sabbath, and on the question of paying taxes. They even conspire with the Pharisees.
I infer that the editor wanted to continue to use the positive term “Essenes” in his own world and not contaminate it with association with Pharisees. Possibly the Roman congregation had Essene intellectual heritage. That heritage could have been from a dissenting branch of the Judean Essenes, or from Essenes in another country. Impossible to know which or when.
The editor now changed “chief priests” to “Sadducees” in Mk 12:18. During the performance, the audience had only seen “chief priests.” The chief priests were important in the play only inasmuch as they were members of the Jerusalem Council and opposed to Jesus. Their doctrines were irrelevant to their role in the play and their sectarian name “Sadducees” was extra, irrelevant information. The editor’s change must have solved a problem in the editor’s world. I propose that problem was the question by a reader of Josephus.
Now to the term in GMark: the “leaven of Herod”. On the return boat trip from Dalmanutha, Jesus says, “beware of the leaven of the Pharisees and of the leaven of Herod” (Mk 8:15). Leaven (or yeast) means exegesis. It is appropriate for Jesus to warn his disciples about the exegesis of the Pharisees (which came from their house intellectuals, the scribes/rabbis). That warning is probably from Mark’s hand, and was spoken in the performance. But the term “leaven of Herod” cannot be taken at face value. We have no reason to think that Herodians had a different set of intellectual authorities! I think that the simplest explanation is that the same editor who added “Herodians” to the polished text added “leaven of Herod.” Why? “Leaven of Herod” = “Exegesis by Judean Essenes.” The editor, I propose, realized that once he had added Herodians to the array of Jesus’s opponents, Jesus should also warn against the “exegesis of Herodians(Essenes).” That term implies that the “Herodians” were exegetes. The extension of the concept of Herodians to exegetes strengthens the case that the editor intended “Herodians” to represent Essenes, who were exegetes.
What do we find in GMatthew and GLuke? In Mk 3:6, the Herodians appear as allies of the Pharisees on the question of healing on the sabbath. The parallel verse in GMatthew (Mt 12:14) does not mention Herodians. However, in GLuke (Lk 6:11) the Pharisees do have allies: “teachers of the Law.” It looks to me like Luke get the idea that the Pharisees had allies from GMark. But Luke removed the now irrelevant (and probably puzzling) term “Herodians” and replaced it with the Pharisees’ real allies in GMark: the scribes/teachers of the Law.
The second mention of Herodians is in Mk 12:13, where they are allies of the Pharisees on the question of paying taxes to Caesar. GMatthew has, mysteriously, retained the Herodians (Mt 22:16). GLuke (Lk 20) has only “spies” sent by the chief priests and the teachers of the Law. Again, Luke has removed the irrelevant Herodians.
Final comment: The changes made by the editor of GMark must have had some historical plausibility. But the editor was not writing the history of Judean sects in the first century CE. We should not treat the mentions of Herodians and Sadducees in GMark as anything other than an editor’s improvised response to a curious reader of Josephus in Rome, at least six decades after the time of the world of the story. We should not, for example, infer that Sadducees regularly engaged in debate with visitors to the Temple. Or that Essenes made political common cause with Pharisees on any issue other than opposition to Mark’s concept of Jesus. Or that Essenes in Judea and Galilee were known as “Herodians.” These proposals remain possible, but GMark is not a historical primary source for them.
Note: A paragraph that describes scholarly proposals for the meaning of “Herodians” can be found here, under the discussion for v. 6.