There was a time when third party candidates would never be considered. That is starting to change. Candidates such as Ross Perot have paved the way for future third party candidates. If the Democrats and Republicans do not start working together and seeing the error of their ways a third Party Candidate will likely make it to the White House in the not so distant future.
The two party system in America is an entrenched reality. It will be extremely hard to change or undo that reality. Powerful elites observe one guiding rule above all else: cling to power as much as possible. The last viable attempt at a third way came with Ross Perot, and that fizzled out after two presidential election cycles.
As the American political system is currently situated, a third party does not stand a chance of establishing itself on a national scale. Third party candidates in presidential elections have not received more than 2% support. People are afraid of "throwing away" their vote and instead choose the candidate they dislike the least. Until the United States implements some sort of instant runoff voting, third party candidates will be unable to gain traction.
No we are a self governing nation and we wouldn't allow anyone to come in and tell us what to do. However for some strange reason we try to go and tell other countries what they can and can't do but to do it to us would never happen it would be a huge war over this.
I do not believe that a third party will ever have an influence in American politics. I think that the America people are way to dedicated to the two party system that currently exists in politics today. There is also just way too much power in botht the Democratic and Republican party.