U.S. elections have not acted as a modern, legally entrenched duopoly since the founding

While America has almost always had two dominant factions at any given time, the nature of that system has changed completely.

For the first century of American history, the two-party system was fluid, unstable, and highly vulnerable to hostile takeovers. The modern, airtight "duopoly" that legally locks out competition is actually a relatively recent invention, developing primarily over the last 130 years. [1, 2]

The evolution of the U.S. election system can be broken down into three distinct phases:

Phase 1: The Era of Party Collapse (1789–1860)

During the first 70 years of the republic, the political market was highly volatile. Dominant parties regularly went bankrupt, dissolved, or were displaced by third-party startups. [1, 2]

• The "Zero-Party" Start: George Washington was famously elected without a party, and the Founders explicitly designed the Constitution hoping to avoid them. [3]
• Rapid Churn: The first dominant pairing—the Federalists and Democratic-Republicans—crumbled after the War of 1812. For a brief moment during the "Era of Good Feelings," America effectively had a one-party monopoly under James Monroe. [3, 4, 5, 6]
• The Whig Collapse: When the system reorganized into the Democrats and the Whigs, it still wasn't a locked duopoly. In the 1850s, the Whig party completely imploded over the issue of slavery. [1, 2, 5]
• The Ultimate Third-Party Success: A brand-new anti-slavery third party—the Republican Party—formed in 1854. Within just six years, this "start-up" party won the presidency under Abraham Lincoln in 1860, permanently replacing the Whigs. [1, 5, 7]