Louisiana is one of those states that does things differently than everybody else.  (Louisiana also tends to change the rules every cycle.)  This cycle, like Washington and California, everybody runs in one primary regardless of party.  However, Louisiana differs from Washington and California in two significant ways.  First, unlike Washington and California, where the primary is simply a primary and the top two advance to the general regardless of whether anybody gets a majority of the vote, the Louisiana primary is actually the general election.  Thus, if somebody gets a majority, they win the election.  It is only if nobody gets a majority that the top two advance to a run-off.  Second, because it is actually the general election and not the primary, the first round of the election is on November 8.  Because of these two features, close elections in Louisiana tend to go to the December run-off.  Thus, if the Senate comes down to Louisiana, we may not know who controls the Senate until mid-December.  Because Louisiana will be an open seat, there is a great likelihood that this race will go to a run-off.