In the United States, the two political parties can almost always be labelled "in power" and "opposition." Over time, since the opposition desires to be "in power," they tend to adopt those policies which have the broadest support not already adopted by the party in power. As such, your general analysis is correct - political parties in the United States as a whole are rarely on the extremities seen in European parliaments.
That said, an analogy is in order here. I live in Washington, DC. When we get 4" or 5" of snow, it's "Snowmaggedon" or a "Snowpolaclypse." Up in Erie, PA, it's a normal winter day. The definition of "extreme far left" or "extreme far right" tends to mean "more wrong than other stupid people in the opposing party." Within the Republican Party, for example - typically the party of low taxes and low government involvement, there is a faction known as the "Tea Party," which is even more strident in its opposition to taxes then the rest of the party. (And I say this as a pretty conservative Republican.) Remember also, that even Democrats view a 39% tax rate as "high," something say a Swede or a Norwegian would laugh at. Paul Ryan, on the other hand, considered to be pretty hardcore, considers a tax rate of 25% to be appropriate for the highest tax rate. Again, not that far.
As to the emergence of third parties - the mere fact that the U.S. has a "First-past-the-post" system tends to discourage them. To be sure, there is typically a "Constitution Party" that advocates low tax rates (like Ryan), and conservative social laws (like overturning Roe v. Wade) that might get 1% of the vote. And, there are the Libertarians who would have us all smoking pot whenever we wanted, and they too might get 1%. The Greens - in the U.S. strictly environmentalist, no Communism here! - are in the same league. There just isn't really an appetite.
The most successful third party in recent memory - Ross Perot's 1992 Presidential bid - actually garnered around 19% of the popular vote. But, in Perot's case, he held very "moderate" views. He wanted to cut the budget, raise taxes, and restructure debt - but overall it was a "take a business approach to the government." Overall, it was a very moderate, "pox on both your houses" approach to politics. And, even with such a strong showing, Perot was unable to make a serious long-term impact on politics, mostly because his best ideas were co-opted by the two established parties.
The lesson to be taken is this - extreme views just don't make a difference in American Politics. They might get token votes, but only the hardest core political junkies know about them. Only in the tightest races (2000 - Ralph Nader) do they even get looked at. Those views that do impact the nation tend to get absorbed into the established parties, so they stay mainstream.
In short, extremism don't really work well in the United States.
