running mate

Short definition from Wiktionary

Noun

1. 1
   A candidate on a joint ticket.
2. 2
   A candidate for the US vice-presidency.
3. 3
   One of a pair of ships with coordinated, often reciprocal, sailing schedules.
4. 4
   One of a pair of horses running the same race for the same stable or owner.
5. 5
   Alternative form of running mate.