Duopoly

http://media.salon.com/2011/09/the_unified_democrat_republican_platform.jpg
du·op·o·ly
d(y)o͞oˈäpəlē
Among Fifty Most Populous Countries, U.S. and Nigeria are the Only Nations with Only Two Parties in the Lower House of a National Legislature
http://ballot-access.org/2015/04/12/among-fifty-most-populous-countries-u-s-and-nigeria-are-the-only-nations-with-two-parties-in-lower-house-of-national-legislature/
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The Two-party system (and Money
Wikipedia: "Modern American politics, in particular the electoral college system has been described as duopolistic since the Republican and Democratic parties have dominated and framed policy debate as well as the public discourse on matters of national concern for about a century and a half. Third Parties have encountered various blocks in getting onto ballots at different levels of government as well as other electoral obstacles, more so in recent decades. There is strong agreement that the United States has a two-party system; historically, there have been _few instances_ in which third party candidates won an election..."
GreenPolicy Siterunner comment: More than a "few instances" -- in the US there have been and are currently hundreds of examples of third party candidates who have won elections and/or have been appointed/confirmed to office at the local and state levels of government.
It is at the federal level and in races for President where the existing ballot access laws, constraints, practices and restrictions act most significantly to limit effective participation by independent third parties/minor parties, although for a number of decades polls have indicated that approximately 30+ percent of US citizen voters consider themselves "independents" and choose not to align themselves with either of the two major parties.
In the US, restrictive ballot access laws act to prevent political parties/candidates apart from the two major political parties (the duopoly in practice)from running and effectively competing in election campaigns... the US is nearly unique in this exclusionary electoral system among modern industrial nations
More on the the political restrictions of ballot access laws:
https://en.wikipedia.org/wiki/Ballot_access
Ballot Access News
Via Richard Winger/Ballot Access News, here are some other US independent party/electoral reform sites which may be of interest:
Project Vote-Smart -- http://votesmart.org/
Project Vote Smart is a citizens’ organization dedicated to serving all Americans with accurate and unbiased information for electoral decision-making. It was inaugerated in 1992 by former US Presidents Jimmy Carter, Gerald Ford, and other leaders. Its webpage offers information about all ballot-listed candidates for all federal and state office.
 
The ACE Project -- http://aceproject.org/
 
An interesting site concerning itself with the “Administration and Cost of Elections”, including issues of fairness and regulatory approaches in various countries. They seem to be almost blind to the ways that third parties in the USA are harmed by campaign finance rules crafted for the problems characteristic of the larger parties, or the ways that third parties would be disenfranchised by various proposed rules, but this is nevertheless a useful resource, particularly for the international comparisons it makes.
Atlas of U.S. Presidential Elections -- http://uselectionatlas.org/
 
A surprisingly complete listing of votes cast in Presidential elections, including numerous third-party candidates and nice maps of vote distribution by state and (on the individual state pages) by county.
 
Center for Voting and Democracy - FairVote -- http://www.fairvote.org/
Folks concerned with alternative voting systems, and related issues, from a moderately leftist perspective. Useful articles describing how better systems of voting and electing actually work.
 
Green Politics, Independent Politics
http://www.greenpolicy360.net/w/Category:Green_Politics
http://www.greenpolicy360.net/w/Category:Green_Party
http://www.greenpolicy360.net/w/Category:Green_Values
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