I took a Congressional Politics Class under a professor Ross K. Baker, who is acknowledged as one of the leading experts on Congress. I got a peek into what happens on the Hill. One instance of insidiousness...
When Representative Luis Gutierrez opposed Nafta during the Clinton administration, he received a call from his sponsors (after recieving one from the president) saying they were unhappy about this decision. Gutierrez noted that there had to have been a concerted effort to call up his sponsors by the party leadership, and considering the bill in question, I wouldn't dismiss it.
The two parties do a lot of consolidating, and this is one aspect of it. If the set ideologies are not followed, than one cannot expect to find little help from the upper-echelons of the party in question. Those outside the two parties aren't simply not heard, they are completely ignored and suppressed. The debates, which were taken over from a neutral third party by the leaders of the two parties after an independent won a significant percent of the presidential vote (at least two digits), are just one symptom.
Some may say all this is natural and normal, personally, I think its a catch 22 which is purposely propagated by the Republican and Democratic parties.