The Collaborative International Dictionary of English v.0.48:

Partisanship \Par"ti*san*ship\, n.
   The state of being a partisan, or adherent to a party;
   feelings or conduct characteristic of a partisan.
   [1913 Webster]

WordNet (r) 3.0 (2006):

partisanship
    n 1: an inclination to favor one group or view or opinion over
         alternatives [syn: partiality, partisanship] [ant:
         impartiality, nonpartisanship]