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Phase 3: The Modern "Cartelized" Duopoly (Mid-20th Century to Present)

The duopoly as we know it today—where the two parties act less like rivals and more like joint-owners of the democratic infrastructure—firmly solidified in the mid-to-late 20th century. [11]

The parties shifted from private civic organizations into heavily state-subsidized entities. Step-by-step, they passed laws to insulate themselves from competition: [12]

1. Primary Elections: In the early 20th century, states began running and taxpayer-funding party primaries. This co-opted the state's administrative power to serve the internal needs of just two organizations. [12, 13]
2. Gerrymandering Technology: In recent decades, advanced computing has allowed the two major parties to draw legislative districts with surgical precision, effectively choosing their voters and locking in permanent incumbency. [14]
3. Debate Lockouts: In 1987, the parties stripped the non-partisan League of Women Voters of its control over presidential debates and created the corporate-sponsored, bipartisan Commission on Presidential Debates, setting up rules (like the 15% polling threshold) that effectively institutionalized a two-buyer market. [10]

Why Duopoly is the "Natural State" of the U.S. System

While the legal barriers keeping third parties out are relatively new, the mathematical pressure pushing America toward two parties has existed since the founding.
Political scientists call this Duverger’s Law. Because U.S. elections rely on Single-Member Districts and Plurality Voting (Winner-Take-All), voters naturally realize that voting for a third party risks acting as a "spoiler" that helps their least favorite candidate win. [15]