What evidence is there that the Herodians were distinct from mainstream Phariseeism? During the New Testament era were the political and religious interests of the Pharisees closely aligned with the Herodians? Or did the nationalistic elements of Phariseeism make them allies?
Part of the last paragraph in entry the Pharisees on New Advent has:


  [T]he extreme faction of the Sadducees, known as the
  Herodians, was in sympathy with the foreign rulers and pagan culture,
  and even looked forward to a restoration of the national kingdom under
  one of the descendants of King Herod. Yet we find the Pharisees making
  common cause with the Herodians in their opposition to the Saviour
  (Mark 3:6; 12:13, etc.).


Herodians | Jewish Encyclopedia are also identified with the Sadducees.