running mate:
(plural: ‘running mates‘; for the right pronunciation, please click here)
(in politics) a candidate/nominee in an election for a lower position/office from the same political party, as in the vice-presidential candidate running along with the presidential candidate;
(in social activities) a person that stays with another person; a companion,
In 2012 presidential elections in the USA, the presidential candidate from Republican Party chose Sarah Palin as his running mate.
Danny, who had high hopes of his future political career, was very much disappointed when the his Party chose him as the vice-presidential nominee, the running mate to a senior female presidential candidate.
Peter is looking for a running mate, as his companion who can speak some foreign tongues and who can withstand the strain of being on the move all the time, in his future adventure to go round the world on foot.!

