The primary elections are certainly internal party affairs, yet the candidates campaign in public for all to hear. Except for the closed caucuses and committee meetings, they campaign in public venues in which, one would assume, Democrats could wander in and ask questions. Is it an artificial distinction to say that participation in the nationally-televised debates should be closed to non-Republicans?

In theory, primaries can be as restricted or open as a party wants. In some places, anybody can vote in one. In others, only registered party members can vote for primary candidates of that party.

The Republican candidates went into the CNN debate voluntarily, so they probably have no one else to blame if a liberal news outlet chewed on them a bit. You might have noticed that the Democrats won't come within five miles of Fox News, so don't talk to me about fear of hostile questions.

CNN carefully selected the questioners from a large number of applicants. We aren't talking about people who "wandered in" from off the street. It isn't so much about who they were as about that information being either deliberately concealed or foolishly overlooked. To be fair, I don't think they identified anybody that clearly, although some (like Norquist) were obvious.

Your point on "activists" is also a fair one. You could probably reasonably define anyone who would go to that much trouble to ask questions at a debate as an "activist" on at least some level. But again, we'll see how many Republicans get to take the microphone at the next Democratic debate.

When the two candidates are selected, and start debating each other, it's open season.