running mate

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All online, a party platform will be drafted, and the field of candidates will be narrowed down to six.Each of those six will choose a running mate ― from a different party.
一切都是通过网络进行的：网上起草政党纲领，将候选人的范围压缩在六个以内，然后六个候选人各自挑选一个来自不同政党的人作为竞选伙伴。
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There is not much he can do about his thin résumé or his lack of foreign-policy and security expertise, though he can mitigate the latter somewhat with an astute choice of running mate.
对于他单薄的简历或者缺乏对外政策以及安全专业知识的缺陷，他能做的已经不多了，尽管对于后者来说，他能够通过选择一位竞选伙伴来弥补这一缺陷。
Then, on Thursday afternoon, after he picked Senator Tom Eagleton of Missouri to be his running mate, McGovern allowed other names to be put in nomination against him during the voting that night.
接着，在星期四的下午，他挑选了密苏里州的参议员汤姆.伊格尔顿作为他的竞选伙伴之后，而在当天晚上的投票中，他又默认了另外几个提名人被拿出来比较。

She became well-known after Republican presidential candidate John McCain chose her as his vice presidential running mate.
Delegates considered several other candidates before they chose him as Roosevelt's running mate.
Sarah Palin emerged from national obscurity in 2008 to become Republican presidential candidate John McCain's vice presidential running mate.

While introducing McCain, Palin sharpened her attacks on Obama and his running mate, Joe Biden.
NPR: With Economy As Focus, Candidates Hit Trail

in the US, a political partner chosen for a politician who is trying to get elected

In an election campaign, a candidate's running mate is the person that they have chosen to help them in the election. If the candidate wins, the running mate will become the second most important person after the winner.