The Herodians distinguished themselves from the two great religious - political parties of the day (the Pharisees and Sadducees) by the fact that they were sincerely friendly to Herod the Great and to his dynasty.
The ruling dynasty that the Herodians supported was begun by Julius Caesar in 47 B.C. when he appointed Antipater I the Idumaen to be procurator of Judea. His son Herod (the Great) began to rule Judea in 37 B.C. Herod Antipas, son of Herod the Great, became tetrarch of Galilee and Perea in 4 B.C. after the death of his father. It was this Herod who jailed and beheaded John the Baptist and who sent Jesus to Pilate after his arrest.
Jesus, early in his ministry, attended a synagogue on the Sabbath where he healed a man's withered hand. The Pharisees who saw the miracle believed Jesus broke the Sabbath because they considered the healing "work." Jesus' trespass of their man-made rules (not God's law) motivated their hard hearts to seek the help of the Herodians to murder him (Mark 3:1 - 6). This is the first plot against Jesus mentioned in the gospels and the first time the followers of Herod are mentioned in the New Testament.
On another occasion the Pharisees wanted to try and trap Jesus into stating something which could be construed as being against the Roman government. Their plan was to send some of their disciples and some Herodians to Jesus and have them ask him a certain "honest" question (Matthew 22:15 - 16, Mark 12:13 - 14). The question was, "Is it lawful to pay taxes to Caesar, or not? Shall we pay, or shall we not pay?" (Mark 12:14 - 15, NKJV). Those who heard his response were amazed!
And Jesus answered and said to them, "Render the things of Caesar to Caesar, and the things of God to God" (Mark 12:17, HBFV).
Why were the Herodians, who were not a religious group, so vehemently against Jesus? One Biblical commentary attempts to answer this question by stating the following.
"All the friends of the family of Herod were opposed to Christ, and ever ready to join any plot against his life. They remembered, doubtless, the attempts of Herod the Great against him when he was the babe of Bethlehem, and they were stung with the memory of the escape of Jesus from his bloody hands" (Albert Barnes Notes on the Bible, comments on Matthew 12:14).
The Pharisees wanted Jesus to prove he was the Messiah by giving them a sign or performing a great miracle (Matthew 12:38 - 40, 16:1 - 4). Herod, who saw Jesus after his arrest, also desperately wanted to see Jesus perform a miracle "on command" as it were (see Luke 23:8).
The Bible states that Jesus warned his disciples about the Herodians. He told them, "Watch out! Be on guard against the leaven of the Pharisees and the leaven of Herod" (Mark 8:15, HBFV). The leaven Jesus warned of was that the Pharisees wanted a heavenly sign and that Herod had long desired to see Jesus perform a miracle on command (Luke 23:8). The leaven was also symbolic of the teachings of the Pharisees and Herodians (Matthew 16:12) and their opposition to the gospel.