WordNet (r) 3.0 (2006):

pressure group
    n 1: a group of people who try actively to influence legislation
         [syn: lobby, pressure group, third house]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "pressure group":
   Black Power, Italian vote, Jewish vote, Polish Power, Rasputin,
   Svengali, VIP, White Power, access, bad influence, big wheel,
   black vote, breakaway group, camp, caucus, court, division,
   eminence grise, ethnic group, faction, farm interests,
   financial interests, five-percenter, friend at court,
   good influence, gray eminence, heavyweight, hidden hand, influence,
   influence peddler, influencer, ingroup, interest, interest group,
   key, kingmaker, labor interests, lobby, lobbyist,
   lords of creation, man of influence, manipulator, minority group,
   offshoot, open sesame, party, political party, powers that be,
   sect, side, silent majority, sinister influence, special interests,
   special-interest group, splinter, splinter group,
   the Establishment, very important person, vested interests,
   vocal minority, wheeler-dealer, wing, wire-puller