There was an interesting discussion a few weeks ago on The Carpetbagger Report about the viability of third parties in the US political system.
Of course, the US system is officially a multi-party system. There are dozens of political parties out there, each with a name, a logo, a platform, elected officers, a newsletter, etc. Several have managed to field candidates in local and statewide elections. Greens, Libertarians and Reformists have had Presidential canidates and some impact during the last 20 years or so. You can find a complete list of US political parties here, here and here. Those lists are fun to browse - there are Nazis and Communists, and everything else imaginable in-between.
But I think that Dr.Biobrain is onto something. A lot of people who join and support third parties think of political parties in European terms. With such a mindset, having Republicans and Democrats clearly dominate the scene seems wrong. But, Dr.Biobrain is correct that these two major parties are not to be thought of as parties in the European sense. Actually, they are equivalents of coalitions. The GOP is a coalition of unofficial right wing parties, while Democratic Party is a coalition of unofficial centrist and center-left parties. Both parties like to think of themselves as "big-tent" parties. The various interest groups within each party are equivalents to individuated parties in the European sense. The only ones without a home are the true Left who tend to vote Green sometimes, or sometimes decide to hold their noses and vote for the lesser of two evils - the Democrats.
The biggest difference between Euro-style multi-party system and the US-style two-coalition system is that in European countries (as well as countries around the world that use the European model) it sometimes happens that unusual coalitions form, in which ideologically opposite groups, out of neccessity, form temporary alliances. This is impossible in the USA. The two large parties/coalition are aligned ideologically.
Here is a taxonomy of center-left or Progressives, as envisioned by George Lakoff:
"From sociological or political perspectives, there seem to be dozens, if not hundreds, of types of liberals and progressives. But from a cognitive perspective, defined by modes of thought, there are just six:One can easily make a mirror-image taxonomy of the conservatives, too. How about: social conservatives, Religious Right, libertarians, supernationalists, ubercapitalists, bigots and authoritarians. Just like the Left groups are unified in ideology as much as they may be diverse in emphasis, so the Right groups are also unified ideologically in spite of often being seen as splintered. Read Dr.Biobrain's post!
1. Socio-economic: All issues are a matter of money and class.
2. Identity Politics: Our group deserves its share now.
3. Environmentalists: Respect for the earth and a healthy future.
4. Civil Libertarians: Freedoms are threatened and have to be protected.
5. Spiritual progressives: Religion and spirituality nurture us and are central to a fulfilling life.
6. Anti-authoritarians: We have to fight the illegitimate use of authority.
All of the types of progressives are right. They all share a commitment to nurturant values, but prioritize different ones. Often this is a source of tension, since a person who strongly identifies with one type may not see how the others are similar. The key to forging unity is to see that each type is a special case of a more general identity grounded in a moral system that links all progressives together."
As a result, both major parties/coalitions fluctuate between strong unity and internal strife, depending on the external events. But it is absolutely impossible to cleave a Right-wing group and get it to ever even consider voting for Democrats. It just does not cross their minds. If they are very unhappy with the way the GOP leadership is behaving at any particular time in history (as they should be right now), they are much more likely to not show up at the polls, or to give a protest-vote to a third party (e.g, the Constitution Party or Libertarians) , then to vote for a Democrat. As far as they can see, Democrats are alien creatures, with alien thought-processes and alien ideas about the world.
This is true on the other side, too. No matter how disgusted we all are with the current Democratic leadership, we will still vote for them, or abstain, or cast a protest vote for the Greens, but voting for a Republican will never cross our minds - from our perspective, they are all sexually-perverse, greedy, sexist, femiphobic, homophobic, racist, xenophobic, supernationalist, pseudo-religious, authoritarian, nasty, irrational, crazy mediavalists.
Thus, a political strategy should completely ignore the core members of the opposing ideology - they are unattainable. One needs to focus on those that can be won over - the independents, the apathetic, the new young voters, and the disgusted core that may rebel and go Green (btw, there is not such thing as a 'moderate'). All four of those groups can be won over only by exciting bold ideas that are true to the core liberal ideology. Triangulation aka 'tacking to the right' is boring and wins nobody over, while alienating the base. Bad idea.
Update: Shakespeare's Sister has just posted an excellent essay on the marginalization of Progressives within the Democratic Party and why that is wrong. A Must Read of the Day (and Year)!