An excellent site. There's so much info here. Great comments.
The two-party arrangement, as well as campaign financing, is at fault in giving us too "me-too" parties. The Dems need not worry about the left because of "loyal Democrats" so they can, as they did big-time under Clinton, screw the working class and the other "loyal Democrats" who have no other place to go.
Third and fourth parties exist in other countries and become power brokers on important policy issues as they align this way and that, but that is not possible in a two-party system. The US system is of course not dictated by the Constitution but has been reinforced recently as the two look-alike parties make it nearly impossible for third parties to participate in the electoral process. Read Ralph Nader's "Crashing the Party" about the procedural difficulties in just getting on the ballot--the petitions (why petitions?) have to be a certain size and color, etc. Kerry legally challenged Nader's ballot access in every state and it's expensive to hire lawyers and counter the challenges. Third party candidates are barred from presidential debates by the Repub/Dem joint committee that runs them. And on and on, the result being that democracy is lost and "loyal Democrats" have to hold their noses and vote for losers like Kerry whose positions differed little from Bush's--as we will always see in a regimented two-party system where every candidate takes a strong position squarely on the fence.
"The United States as envisioned by the Founding Fathers no longer exists. We were simply the Americans who happened to be here when it died."
Phid nailed it, as usual.