running mate 
noun chiefly North American
1An election candidate for the lesser of two closely associated political offices.His running mate as vice presidential candidate would also be announced....
- He is his vice presidential running mate for the election in November.
- He is also the son of the vice-presidential running mate of a SEP presidential candidate.
2A horse entered in a race in order to set the pace for another horse from the same stable, which is intended to win.He has a record of 2.15 with a running mate.