From Kevin Drum:
As for third parties, I'll say only this: in 1980, when I was 22, I voted for John Anderson. That sure was stupid. Eight years of Ronald Reagan because Jimmy Carter didn't quite meet my idealistic standards of excellence for presidents. I've never made that mistake again.This is the issue with first past the post systems -- splitting up the vote from one coalition can lead to the other one being elected. Just ask Canada about majority Conservative governments with a minority of the popular vote.
But it is worse in the United States of America. For a law to be passed, it needs to pass the house, pass the senate, and then not be vetoed by the president (or have the veto overridden). With two partisan parties, it already leads to a lot of gridlock. Imagine if you needed to build coalitions in both the house and senate?
Now, some degree of gridlock might be a feature and not a bug if one is distrustful of government. But there is probably a limit to how unresponsive we want government to be to actual problems, including those of bad government policy.
So third parties are both a chance to push the opposition past the post, but there isn't really a vision as to how a third party would work without one of the old parties collapsing. And I am not sure how that reforms the parties -- it just shuffles the coalitions and puts new labels on them.