Anyone who complains about how a robust third party would make the American electoral system better need only look to the UK for a quick refutation. First-past-the-post systems (in which the person in each district with the highest vote-total wins that district) do not mix well with coalition-style governments with several major parties. The Conservatives won only 43% of the vote in the UK nationally on Thursday, whereas the three more liberal parties (Labour, SNP, and Liberal Dems) won about 47%. However, the Conservatives won about 58% of the seats. This is because they were the largest party in a large number of district, with the liberal vote in those districts split between the more liberal parties.
Part of the problem in the case of the UK is that Labour has become so radical as to render it incompatible with Liberal Dems (who are most definitely not socialists). District-based first-past-the-post systems naturally tend to two parties, because any parties that can combine will have a significant advantage over those that cannot. Of course, this might be reason to favor proportional systems, where seats are awarded to parties as a function of the overall popular vote, over district-based systems. However, if you are in a first-past-the-post system, then you should embrace a system of large moderate parties. If you don’t, then eventually the other side will…