Definition of "oppositionist"

They felt that the fellow was a quockerwodger rather than a politician, an againster rather than a true American, an oppositionist in fundamental things rather than a sincere patriot.

1944, Morris Allison Bealle, Washington Squirrel Cage, page 9

I see your marriage as a move to establish links with the oppositionist Russian intelligentsia.

2015, Rosemary Sullivan, Stalin's Daughter, page 137