Unionist rhymes with:
communist, abolitionist, accompanist, antagonist, arsonist, botanist, calvinist, chauvinist, communist, conservationist, contortionist, determinist, exhibitionist, expressionist, feminist, illusionist, impressionist, isolationist, jansenist, neoplatonist, obstructionist, organist, perfectionist, platonist, protagonist
Unionist sounds like:
umscheid, umstead, unangst, unannounced, unassisted, unchecked, uncoat, uncoated, uncouth, unctad, uncut, unhinged, unionized, unissued, unjust, unmanaged, unmasked, unmixed, unquote, unsaid, unscathed, unseat, unseated, unset, unstated, unsteady, unsuited, unswayed, unused, unwashed