I think the lesson of Howard Dean in the last election is that the Internet is capable of creating one or two new parties. Dean rode the wave of a group of people on the Internet who were looking for a spokesperson.
The Dean people got co-opted by the Democratic party because the Deaniacs hated President Bush more than they hated military action in Iraq.
That won’t happen again. Next time the Internet will give the Lefty Fundamentalists enough organizing power and money (small donations) to form a new party. They probably will. This time the party line Dems won’t be willing to let Michael Moore be the key figure at their convention.
The same possibility exists on the Fundamentalist Right that is dissatisfied with the Republican “cultural” agenda. The right supported the twerp from Texas, Ross Perot, in 1992 which resulted in a defeat for Bush 41. The Internet will make it easier to create a party in the future. Only the chance to loss an election will mitigate the strength of the new Right Wing religious party.