I hadn’t heard of the term before reading this post. Diegogarcity suggests that I will start seeing it everywhere. A little bit of googling turned up several hits to newspaper headlines, all from 2012. My googling was rather sketchy but here is what I found FWIW.
Interestingly, none of the articles using the term bothered to define it.
The term seems to be used pretty loosely to mean pretty much any person who gives speeches or makes presentations on a candidate’s behalf. For example, Sandra Fluke is referred to as an Obama surrogate, and she is certainly not a seasoned politico. To summarize, her primary claim to fame is that she spoke in support of the idea that an employee of an employer who is associated with a religious organization, such as a hospital affiliated with the Catholic church, should have birth control covered under their employer-provided health insurance plans. This prompted a pundit (Limbaugh) to refer to her as a “slut” on his national radio/TV show, generating something of a backlash. Fluke spoke at a major campaign event, and made presentations at other (much less well attended) events. There is more to her story, of course, but the main point is that she is a public figure in only a minimal sense, and is not, AFAIK, a major player in the current political campaign.
Other people referred to as “surrogates” in a political context (in news stories) include Bill Clinton, Rudy Giuliani (mayor of New York at the time of the September 11, 2001 terrorist attacks), Michelle Obama, Ann Romney, governors of various states, and some people who i am inferring are relatively little known, but well-spoken, members of congress.
If the term “surrogate” can be applied to anybody from Bill Clinton to Sandra Fluke to a whip-smart junior member of the legislature, then it doesn’t seem to be particularly focused in meaning.