WordNet (r) 3.0 (2006):

running mate
    n 1: a nominee for the lesser of two closely related political
         offices

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "running mate":
   also-ran, aspirant, baby kisser, buddy, candidate, chum, comate,
   companion, comrade, crony, dark horse, defeated candidate, dud,
   favorite son, hopeful, lame duck, office seeker, pal,
   political hopeful, presidential timber, stalking-horse