Which laws would those be?
Each state has laws determining who can and cannot be on the ballot. Some states its easy...others make it hard. Florida, for example, is especially hard on Independent candidates
But, the way the debates are run is an even bigger problem. The limiting of 3d party and independents in the debates really smells...and reeks of unconstitutionality
Just a quick search - and I picked through a lot of them without reading too deeply. State laws are restrictive towards getting a third party on primary ballots. And that’s where any third party hoping to compete will have to begin.
http://rangevoting.org/BallAccess.html
http://digg.com/2008_us_elections/Ballot_Access_Laws_Discriminate_Against_Third_Parties
http://www.ncopenelections.org/
http://opinionator.blogs.nytimes.com/2006/09/27/third-party-uninvited/
http://www.natural-law.org/platform/democracy.html