From The Gospel of Matthew by Curtis Mitch and Edward Sri, commenting on Matthew 22:15-16:
The evangelist informs us that the Pharisees are up to no good. They are devising a trap to ensnare Jesus in his speech. This time they team up with the Herodians, who are political supporters of the Herodian dynasty and its cooperative relationship with Rome. To put it mildly, these two groups are neither friends nor allies of each other. The Pharisees are religious patriots, bitterly opposed to Roman rule, whereas the Herodians are content to work together with the Gentile powers that be. The present alliance is made solely for the purpose of bringing down the Messiah.