The word "partisan" comes from French (partisan) and Italian (partigiano) roots, meaning "of a part or a  or part or party", ultimately from the Latin  pars (part), evolving from a term for a faction member to a zeaalous supporter, then to a guerrilla fighter (a "party" of "irregular troops),   and also to a type of polearm weapon (partigiana).  Its journey reflects its core idea: being part of a group, whether a political faction, military unit, or cause. 

According to the Oxford English Dictionary (OED), the word "partisan" (referring to a supporter of a party/cause) first appeared in English shows evidence from as early as 1542, with the earliest written evidence found in a 1555 translation by Richard Eden.  The military sense (a fighter in a detachment) emerged later, around the 1690s and the adjective form, initially in a military context, comes from 1708. 

Examples of "partisan" used in a sentence: 
"So what was it that seemingly turned him into a Democratic partisan?" 
"Special interest groups and partisan politics have blocked all efforts at reform since 1986." 
"The partisan warfare flamed up most fiercely in the latter days of October in Russia." 
"He then spent the better part of World War II as a partisan with a group of other fugitives."
"Alexander Mackenzie, is decidedly partisan. Meanwhile an active and bitter partisan warfare opened." 
"And then most neutrals will become partisans for the team that give them more aesthetic pleasure."
"You feel it from the hugely partisan support the team are receiving, but it is also there in the desire and focus of their team."

Examples of "partisanship" used in a sentence:
"Politics and partisanship came close to taking his life." 
"These wars pass into history under a cloak of partisanship."
"They promoted partisanship above respect for the popular will."
"Her reliably fierce partisanship was the least of the qualities required for a divided country." 

Just the facts:

PARTISAN Definition & Meaning - Merriam-Webster

PARTISAN Synonyms: 178 Similar and Opposite Words | Merriam-Webster Thesaurus

Love the reference to The Santa Clause, and I love the holiday icon, @Myr