Look at any failing democracy and you will find constitutional hardball. In postwar Argentina, when President Juan Perón encountered Supreme Court opposition, his congressional allies impeached three of five justices on grounds of “malfeasance” and replaced them with loyalists. In 2004, when Venezuela’s high court proved too independent, congressional allies of President Hugo Chávez added 12 seats to the 20-member court and filled them with loyalists. Both Perón’s and Chávez’s court-packing schemes were legal, but they nevertheless destroyed judicial independence.

Norms of forbearance have not always been strong in the United States: They were weak in the republic’s early years and they unraveled during the Civil War. But for most of the 20th century, Democrats and Republicans accepted each other as legitimate and exercised power with forbearance.

There were no partisan impeachments or successful court packing. Congress routinely funded the government, obstructionist tools like the filibuster were used sparingly, and the Senate used its power of advice and consent with prudence, routinely confirming qualified nominees. There were instances of executive overreach (Franklin Roosevelt, Richard Nixon), but the most egregious abuses were checked by Congress and the courts.

History suggests, however, that democratic norms are vulnerable to polarization. Some polarization is healthy, even necessary, for democracy. But extreme polarization can kill it. When societies divide into partisan camps with profoundly different worldviews, and when those differences are viewed as existential and irreconcilable, political rivalry can devolve into partisan hatred.

Parties come to view each other not as legitimate rivals but as dangerous enemies. Losing ceases to be an accepted part of the political process and instead becomes a catastrophe. When that happens, politicians are tempted to abandon forbearance and win at any cost. If we believe our opponents are dangerous, should we not use any means necessary to stop them?

This is how democracy died in Chile. Before the 1973 coup, Chile was Latin America’s oldest democracy, buttressed by vibrant democratic norms, including a well-established “culture of compromise.” Chileans liked to say that there was no political disagreement that could not be settled over a bottle of Chilean cabernet. But beginning in the 1960s, Chile’s culture of compromise was shattered by Cold War polarization. Mutual toleration eroded, and political parties eschewed forbearance for a “win at all cost” strategy. Chilean democracy fell into a death spiral, culminating in a bloody coup. (The intervention of the United States accelerated but did not cause this death spiral.)

Could it happen here? It already has. During the 1850s, polarization over slavery undermined America’s democratic norms. Southern Democrats viewed the antislavery position of the emerging Republican Party as an existential threat. They assailed Republicans as “traitors to the Constitution” and vowed to “never permit this federal government to pass into the traitorous hands of the Black Republican Party.”