Ben Jacobs, The Daily Beast:

The problem is that Mississippi has long operated as a state that functionally had an open primary. The only limitation in case of a runoff is that a voter can’t participate if he has already cast a ballot in the other party’s primary. Thus, a voter who voted in the Democratic primary on June 3 can’t participate in Tuesday’s runoff. But, with the exception of rogue operators like Brown, the law has long been more honored in the breach than the observance—-particularly in a state like Mississippi, where Democrats still dominate at the local level in many areas. Even a diehard Republican like McDaniel cast a ballot in the Democratic primary in 2003——his campaign has declined multiple requests for comment as to whether he voted for the incumbent Democratic governor, Ronnie Musgrove, in the general election that year and prodded Cochran spokesman Jordan Russell to note that, by the logic of McDaniel’s supporters, the conservative diehard either “broke the law or voted for Ronnie Musgrove that November in the general election.”
http://www.thedailybeast.com/articles/2014/06/24/the-mississippi-republican-s-african-american-voter-intimidation-question.html