Open and Closed Primaries

States like NH hold open primaries, anyone can vote in either party's primary.  Other states like Massachusetts hold closed primaries, you have to be a registered party member to vote in the party primary. 
   Arguments for closed primaries are thus.  An American political party is more than just a bunch of voters.  The party stands for things and politicians who campaign under the party banner are expected to support their party on all levels.  Elected politicians are expected to vote the way the party leadership calls for, even if they themselves are against the party position.  In which case, it makes sense for the selection of nominees be limited to party members, in order to insure that the nominee thinks the way the party rank and file do.  Allowing independents and opposition party people to vote in party primaries dilutes the party members vote and allows the election of wishywashy or even hostile thinking nominees.
   The strongest argument for open primaries occurs in one party states.  In a solid red or solid blue state, winning the primary is equivalent to winning the general election.  In solid blue Massachusetts, winning the democratic primary means you will take office a few months later.  So members of the opposition party cry out for votes in the only election that really matters, the dominant party primary.