History of ancient Israel and Judah
During the Hasmonean period, the Sadducees and Pharisees functioned primarily as political parties (the Essenes not being as politically oriented). The political differences between the Sadducees and Pharisees became evident when Pharisees demanded that the Hasmonean king Alexander Jannai choose between being king and being High Priest in the traditional manner.  This demand led to a brief civil war that ended with a bloody repression of the Pharisees, although at his deathbed the king called for a reconciliation between the two parties.  Alexander was succeeded by his widow, whose brother was a leading Pharisee.  Upon her death her elder son, Hyrcanus II, sought Pharisee support, and her younger son, Aristobulus, sought the support of the Sadducees.