1) In America just the Democrats and Republicans reign supreme at a political level.
2) In the 2008 election no other party will have any chance of success. Why?
3) Instinctively Americans will almost to a person only vote for either the Democrats or Republicans as this is the way they do it.
4) Over the years only these 2 parties have managed to build up sufficient funds to fight a national campaign at the level required for success. Also over the years only these 2 parties have managed to build up sufficient links to big business etc to acquire funding and support. The people of K Street will almost certainly only target men and women who fight for these two parties as any other party has no chance of winning.
5) Historically, the other parties have no chance of success. The Reform Party under Ross Perot in 1992 and 1996 had access to huge sums of money but failed - so money is not a definite pointer to success.
6) The American election system - winner-takes-all - guarantees that any third, fourth party etc has no chance of winning. To succeed in the national election, you simply need a majority of Electoral College votes. Only the Democrats and Republicans have any chance of gaining this. As such anyone who votes for an alternate party is effectively wasting their vote.
7) The American system allows both parties to adopt and shape their policies as and when they like. Therefore any decent policy taken on by a third party can be usurped by one of the big two - and that is acceptable in US politics. Therefore by adopting a third party's policies, the 'big two' can move forward with policies that might be part of a third party's strategy for success - and steal its clothes.
8) The media - a vital part of US politics - will only be interested in the Republicans and Democrats thus starving any other party of the oxygen of publicity. Likewise those who donate money will only be interested in likely winners - and that will only come from the Democrats or the Republicans.
9) The highly expensive campaign that is now all but mandatory for an American election campaign is beyond the financial capabilities of most parties - though whatever guise the Reform Party takes in 2008, it will not be short of money.
Any 'success' by alternates in US politics has always been short-lived. The 2 Independent Senators in the 2007 Senate both came from the established parties and left as a protest. Would they have been voted in an Independent?