Other. Actually, none of the above. Until structural changes are made to the US political system, the two party monopoly will continue and the current two dominant political parties will continue to dominate. Alternative political parties can only advance so far before having their platforms co-opted right out from under them by one of the two dominant parties. That is why "third" parties never last long in America. The Populists of the late 1880's and early 1890's are the best example. Single member districts; taxpayer funded partisan primaries (nearly always for Democrats and Republicans only); "non-partisan" debates commission that excludes everyone except Dems and Reps; news media which excludes alternative candidates etc. Not a pretty picture for minor parties.
I asked which has the best chance out of all of them, not whether it would happen or not. I'll grant you it's an uphill battle. The best chance for any of them to rise would be the collapse of one of the major parties, or something that severely weakens them at least. You must remember, the Republicans were a third party once, so a third party coming into power is not out of the question.
Just to give my opinion on what would happen if one of the major parties collapsed, it would first depend on which one collapsed. The Dems look more likely at this point, so I'll start with them - I'd say a number would go to the Green Party, definitely those on the far left, but the economic moderates might go for the Libertarians, since they will likely agree on social policy. If the Republicans collapsed on the other hand, it would be a much different split - The Constitution Party would definitely benefit, but the less socially conservative Republicans would likely join the Libertarians. Of course, if either party collapses, I think we'd see even the non-collapsed party losing members to the growing third parties. Of course, this ignores the possibility that new parties would form from the ashes of the old. Might be my bias speaking, but I think the Libertarians are probably in the best position to capitalize on a collapse, we're probably the best organized in terms of doing it as well.
