non-partisan (adj.)

also nonpartisan, "not based on, belonging to, or loyal to any particular political faction or party," 1872, American English, from non- + partisan (adj.) "pertaining to a (political) party."

FIRST POLITICIAN: Who's backing this non-partisan candidate?

SECOND POLITICIAN: The non-partisan party.

[Life magazine, Sept. 29, 1927]

As a noun, "a non-partisan person," from 1888.