Are there legal obstacles to an elected official appointing a special prosecutor to pursue his/her opponent?
<p>The current US Presidential campaign has had a lot of mud-slinging and accusations back and forth about violations of the law by the opposing candidate.  One has explicitly stated plans to (if elected) instruct the Attorney General to appoint a special prosecutor to go after the other, some of whose supporters would likely support a similar plan if their candidate wins.   (<em>Edit: The candidate overtly making this pledge won the election.</em>)<br>
<strong>Are there any legal barriers to doing this?  If so, what are they?</strong></p>

<p>In general public policy terms, people are fed up with special treatment under the law for elites, but the US might block itself from getting high quality candidates in the future if it establishes a pattern of sending candidates to jail after losing elections.   <em>(Please, no comments on the quality of candidates in the present election.)</em></p>

<p><a href="http://time.com/4524459/presidential-debate-donald-trump-special-prosecutor/" rel="nofollow">TIME</a> says it would be a first, so there's no clear precedent.  <a href="http://www.cnn.com/2016/10/09/politics/fact-check-presidential-debate/index.html" rel="nofollow">CNN's fact-checker</a> says there are no obstacles (beyond the obvious requirement of getting elected first):</p>

<blockquote>
  <p>The US Attorney General does have the authority to appoint a special prosecutor, according to Stephen Vladeck, a CNN legal contributor and law professor with the University of Texas School of Law. Such action can be taken when the Attorney General determines that a criminal investigation is warranted and that an investigation by the Department of Justice would present a conflict of interest or "other extraordinary circumstance" and that "under the circumstances it would be in the public interest to appoint and outside Special Counsel."</p>
</blockquote>

united-states criminal-law prosecutorial-discretion executive