Since 1988 the CPD has been in control of the debates that are televised in 2000 they created a rule that the party must have 15% support to be included within the debate. The fault with this rule is that the debates are where a lot of people choose which party and/or candidate to support.
To read more about the CPD please look here
http://en.wikipedia.org/wiki/Commission_on_Presidential_Debates