No the Constitution does not say that only R's and D's have a right to be heard, but let me ask you a question:
If I go to Arizona and I declare myself as a Presidential Candidate, why can't I be up there, debating with President Bush and Senator Kerry? If one-hundred others do the same, why can't they?
The reasons are the rules of eligibility. In order to prevent the anarchy of the above, the rules of the debate commission are in force.
This thread seems to be assuming that the rules of the debate are prejudicial against this particular candidate. If so, what are the rules and how is it prejudicial?