Source NYT 1/27/18: "How Wobbly is our Democracy?"
Written by Steven Levitsky and Daniel Ziblatt. Democracy is reinforced by two main principles. First, mutual toleration - we assume that the other side also sincerely cares for the country and will do a decent job of governing. Examples of this from history: first, Chile in the 1960's became increasingly polarized over Cold War issues of capitalism versus socialism. Each side adopted "win at all costs" mentality which eventually resulted in their 1973 military coup. In the US in the 1850's, Democrats and Republicans felt the other was an abomination. Joanne Freeman of Yale has shown there were 100 instances of violent acts on the floor of Congress between 1830 and 1860 (the caning of Republican Charles Sumner being the most infamous).
Second: forbearance - those in power don't use it to win at all costs. Instead, they show restraint or forbearance in the use of power. For example, a President without forbearance could change the size of the courts and then pack the new seats with his cronies. Or a President without forbearance could use their pardon power to excuse all types of transgressions. A Congress without forbearance could shut off funding and stop the government. Or refuse to consent to executive appointees. Examples from history: Juan Peron in Argentina had three of five Supreme Court justices impeached and removed. In 2004, Hugo Chavez in Venezuela added 12 seats to a 20-person Supreme Court and packed the new seats with his supporters.
Our current polarization is more about race and religion than politics. From the article: "This is not a traditional liberal-conservative divide. People don’t fear and loathe one another over taxes or health care. As political scientists have shown, the roots of today’s polarization are racial and cultural. Whereas 50 years ago both parties were overwhelmingly white and equally religious, advances in civil rights, decades of immigration and the migration of religious conservatives to the Republican Party have given rise to two fundamentally different parties: one that is ethnically diverse and increasingly secular and one that is overwhelmingly white and predominantly Christian. And white Christians are not just any group: They are a once-dominant majority in decline. When a dominant group’s social status is threatened, racial and cultural differences can be perceived as existential and irreconcilable. The resulting polarization preceded (indeed, made possible) the Trump presidency, and it is likely to persist after it."