At this point, the general belief is that US is a two-party country because in addition to the FPTP for congress it has also the winner-take-all executive presidency (not quite FPTP, but close enough). So, DL applies nationwide and district by district, creating a particularly strong Duvergerian force. The reason I call it a "belief" is that US is pretty much the only such country, which is both presidential and FPTP, so proper testing of this belief is impossible.
plus encouragement of polarisation of voters at the point of registration, and horrendous ballot access restrictions.
anyway, back on topic, and my beloved party is having another of its periodic attempts to get Labour, LibDems and Plaid interested in a Progressive Alliance to keep a hypothetical Con/DUP/UKIP "Brexit Coalition" out of office, which will: a) never happen b) only really help labour and c) alienate half our supporters if we hand No10 on a plate to the War Criminals' Party
http://www.newstatesman.com/politics/staggers/2016/06/green-party-calls-labour-lib-dems-and-plaid-cymru-form-progressive
