Definition of Running Mate
What is a "running mate"? What is the definition of the term "running mate"?
As it relates to the US Presidential election, a "running mate" is the person who is the presidential nominee's candidate for Vice President.
For instance, Sarah Palin was John McCain's "running mate" in the 2008 US Presidential election, as Palin was McCain's choice for Vice President, should he have beaten Barack Obama. On the other hand, Joe Biden was Barack Obama's running mate in the 2008 US Presidential election and became Vice President in January of 2009.

In some cases, a running mate will go on to become a Presidential candidate themselves. For instance, Al Gore served as Bill Clinton's Vice President for eight years, and then ran for President himself in 2000 (where he eventually lost to George W. Bush).
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Davemanuel.com Articles That Mention Running Mate:
The Plagiarism Scandal That Ended Joe Biden's 1988 Presidential Campaign
A Look Back at Richard Nixon's Massive Victory Over George McGovern in 1972
Hillary Makes It Official: It's Clinton/Kaine
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Donald Trump Confirms Mike Pence as VP Running Mate