alt Hacker NewsNot a direct cause but popped into my head:
Previous to 1988 the League of Women Voters[0] handled presidential debates. A fully independent outside organization.
Since then, the Commission on Presidential Debates[1] set rules for admittance to president debates. The CPD was founded jointly by Republicans and Democrats and is controlled solely by both parties.
At best, there _appears_ to be a large, gaping conflict of interest when it comes to admitting candidates to presidential debates. In 1992 Ross Perot was invited to the debates as a third option. In 1996 Clinton and Dole successfully argued for Perot to be excluded from the debates as he had no "realistic chance to win" [2]. Perot aside, what happened was downright anti-democratic and further enforced the two party system.
Now that I'm on this...I'll do another example of this abuse of power. Candidates from third parties have been arrested for protesting outside presidential debates [3,4]. Even if the protests broke the law, arresting opponents for asking to be given a podium to speak at feels bad.
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[1] https://en.wikipedia.org/wiki/Commission_on_Presidential_Deb...
[2] https://www.washingtonpost.com/archive/politics/1996/09/18/p...
[3] https://www.theguardian.com/commentisfree/2012/oct/18/jill-s...
Beyond that, a quirk of the American system in many states is registering your party affiliation at the time you register votes so that you can participate in primaries. When you've got >45% of registered voters identifying as being a supporter of one of the major two parties closely enough to add it to their name on the electoral roll, it's not exactly an encouraging environment to break that status quo
Other majoritarian democratic systems often also converge into two party (or two and a bit party, or two parties per region systems) but few seem to normalise voting for the same party every time in quite the same way.