running mate

a candidate for an office linked with another and more important office, as for the vice-presidency.

a horse entered in a race in which another, more important, horse from the same stable, or belonging to the same owner, is run.

a close companion.

Origin of running mate

An Americanism dating back to 1865–70

Running-Mate Question Allows Candidates to Flatter Each Other If ever there was an “Are we in high school?”

GOP’s Most Awkward Debate Moments: Crying, Flip-Flops, More (Video) |The Daily Beast |February 23, 2012 |DAILY BEAST
She and George Pataki did not know each other when he asked her to be his running-mate.

Return of the Health Care Villains |Michael Kinsley |March 18, 2009 |DAILY BEAST
She took good care to choose Désirée for her "running-mate," and as they were both fleet of foot they scored considerably.

The Jolliest School of All |Angela Brazil
It is rumored that from the state of sisal and henequen he will pick his running-mate.

Diplomatic Days |Edith O'Shaughnessy

Why Madero insists on that running-mate we don't understand.

Diplomatic Days |Edith O'Shaughnessy
She was treated, accordingly, as some exquisite chorus-girl or cabaret-pony that he had selected as a running-mate.

We Can't Have Everything |Rupert Hughes

running mate

US a candidate for the subordinate of two linked positions, esp a candidate for the vice-presidency

a horse that pairs another in a team