running mate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The candidate or nominee for the lesser of two closely associated political offices.
• noun A companion.
• noun A horse used to set the pace in a race for another horse.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A candidate on a joint ticket who is not the primary candidate on that ticket.
• noun One of a pair of ships with coordinated, often reciprocal, sailing schedules.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a nominee for the lesser of two closely related political offices