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In politics, a partisan is a committed member of a political party. In multi-party systems, the term is widely understood to carry a negative connotation - referring to those who wholly support their party's policies and are perhaps even reluctant to acknowledge correctness on the part of their political opponents in almost any situation. Partisanship can be affected by many factors including current events, figureheads (presidents), decisions, and even location.
For example, in Commonwealth realms, the monarch is seen as being distinctly non-partisan and thus is vested with certain powers to form or dissolve governments when there is a democratic impasse. This is in contrast to professional politicians who are expected to push for their party's interests.
In the United States, the meaning of the term has changed dramatically over the last 50 years. Before the American National Election Study (described in Angus Campbell, et al., in The American Voter) began in 1952, an individual's partisan tendencies were typically determined from their voting behavior. Since then, "partisan" has come to refer to an individual with a psychological identification with one or the other of the major parties.
See also 
The dictionary definition of partisan at Wiktionary