WordNet (r) 3.0 (2006):

speak for
    v 1: be a spokesperson for; "He represents the Government's
         position"

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "speak for":
   act for, advocate, allege in support, answer, answer for,
   appear for, argue for, assert, back up, champion, commission,
   contend for, counter, defend, deputize, espouse, front for,
   maintain, make a plea, pinch-hit for, plead for, rebut, refute,
   reply, represent, respond, riposte, say in defense, speak up for,
   stand in for, stand up for, stick up for, substitute for, support,
   sustain, understudy, uphold, urge reasons for