A person who belongs to or promotes the interests of a particular faction rather than the larger group or organization.
From factional + -ist (person practicing/supporting something). Emerged in 19th-century political analysis to describe those who put faction above unity.
Calling someone a 'factionalist' is basically an accusation — you're saying they care more about their group winning than about the common good or the whole organization's success.
Complete word intelligence in one call. Free tier — 50 lookups/day.