I interviewed Suzanne Mettler, a professor in the government department at Cornell University, and Robert C. Lieberman, the Krieger-Eisenhower professor of political science at Johns Hopkins University, about their new book, “Four Threats: The Recurring Crises of American Democracy.” Their responses have been lightly edited.

H.F.: The book argues that contrary to general belief, U.S. democracy is not very stable and that we have seen repeated “backsliding” in the past. What is backsliding, and why do we pay less attention to it than we ought?

Mettler and Lieberman: Think of democracy as a continuum rather than an on-off switch. At any given time, a country can be more or less democratic, depending on how close it comes to meeting a basic set of standards — holding free and fair elections, upholding the rule of law, recognizing the idea of legitimate opposition and protecting the integrity of rights. Over time, regimes can move along the continuum in either direction, either toward more complete democracy or in the other direction, the process known as “democratic backsliding.” Even if we recognize that the United States was by no means a complete democracy at its founding, because so many were denied rights and excluded from membership in the political community, many Americans regard our history as a story of progressive democratization, steady movement toward the fulfillment of a democratic ideal. Our exploration of American history reveals that this is not true, that American democracy has been far more fragile and prone to backsliding than is commonly understood.

What are the four factors that you see as potentially undermining U.S. democracy?

Scholars who study the rise and fall of democracy elsewhere recognize four conditions that pose threats to the sustainability and survival of democracy: political polarization; conflict over who belongs as a member of the political community (particularly along lines of race, ethnicity and national origin); high and growing economic inequality; and excessive executive power. These are the four threats of our title. Each threat alone can pose a danger to democracy. In the 1790s, for example, polarization between Federalists and Republicans threatened to tear the new republic apart, and in the 20th century, the growth of presidential power posed a danger as well. But when multiple threats combine, the danger grows particularly acute. When three threats — polarization, conflict over membership and economic inequality — combined in the 1850s, the nation devolved into a devastating civil war. In the 1890s, these same three threats precipitated the disenfranchisement of millions of African Americans. Today, for the first time in American history, we face all four threats at once.

How are they manifesting right now?

The confluence of all four threats today means that they are combining and interacting in particularly dangerous ways. Polarization has been on the rise since the 1980s, dividing Americans into hostile “us vs. them” camps. Conflict over inclusion and status, particularly regarding race and immigration, increasingly divides the parties. Economic inequality has been escalating since the 1970s, prompting the affluent to mobilize politically to protect their wealth from those who seek greater redistribution. For nearly a century, presidents of both parties have claimed greater power for the office and left it behind for their successors, increasing the opportunity and temptation to use power for personal or partisan aims.

Although the threats have been gathering steam for decades and have been on full view during the Trump presidency, events in the past few months have punctuated them even further. The coronavirus pandemic and the economic crisis it precipitated have dramatically exposed partisan, economic and racial fault lines. Americans of color have disproportionately been victims of the virus. The pandemic-induced recession has exacerbated economic inequality, exposing the most economically vulnerable to job losses, food and housing insecurity and the loss of health insurance. And partisan differences have shaped Americans’ responses to the pandemic. Even the simple act of wearing a mask has become a partisan symbol. The Black Lives Matter protests that erupted after the police killing of George Floyd have further highlighted the deep hold that systemic racism has long had on American politics and society.

People don’t usually think of Presidents Franklin D. Roosevelt and Richard M. Nixon as having much in common. Yet you find that Roosevelt’s policies paved the way for some of Nixon’s most controversial innovations. How did that happen?

Executive aggrandizement was one of the key story lines of American politics in the 20th century. Franklin Roosevelt assumed the presidency at a moment of deep political and economic crisis, and many Americans expected him to assume dictatorial power emulating leaders across the Atlantic. Although he did not take that approach, Roosevelt left the presidency much stronger than he found it — with potent tools that, in different hands, could be weaponized to serve a particular president’s personal or political aims. Those developments paved the way for Nixon’s abuses of power a generation later.

You describe a “deeply disturbing pattern” in which across various crises “political leaders effectively preserved American democracy by restricting it.” How has that history shaped modern U.S. democracy and the demands from groups such as Black Lives Matter for change?

With all four threats present, we now face a daunting challenge to democracy. One of the tragic historical patterns we uncovered is that often, preserving democracy for some has entailed perpetuating and expanding racial hierarchy and exclusion, a fundamental compromise of democratic values. In some cases, as in the 1790s and the 1930s, political leaders shared a tacit agreement to leave racial hierarchy intact. In other eras, polarized parties divided explicitly over race. When the party that favored a restoration of racial hierarchy prevailed, as occurred in the South in the 1890s, the consequences for democracy were devastating. Although the parties are divided over race today, however, more Americans than ever are voicing opposition to the persistence of racism that has long diminished our democracy, suggesting that the balance of forces may be shifting in a more pro-democracy direction.