In the 1990s, when liberal democracy seemed to have triumphed over its Cold War rivals, Americans and West Europeans sought to understand the pathologies of the ethno-nationalists, neo-Nazis, demagogues and authoritarian populists of Eastern Europe to better assist their assimilation into the new world. History had ended, and the U.S. model was the future.

Less than two decades later, Westerners are again studying the underlying forces of fascism in an effort to save liberal democracy from an authoritarian tide that has swept from Moscow and Budapest to Ankara and Hong Kong. Nationalist and far-right parties have won double digit-shares of the vote in a dozen European countries. Undemocratic regimes such as China, North Korea and Saudi Arabia have bolstered their positions. And the United States is led by a president who flouts democratic conventions, unleashes federal agents on peaceful demonstrators, and assails independent institutions and checks on his power.

Scholars have analyzed threats to democracy from various angles. Princeton political scientist Jan-Werner Müller has argued that populism, often presented as an appeal to the broad interests of the people, in fact rejects pluralism and veers toward authoritarianism that excludes people deemed unacceptable. In his latest book, “The New Despotism,” John Keane, a professor at the University of Sydney, takes aim at the political tools that modern authoritarians have mastered to assert power.

Now Eric Posner, a professor at the University of Chicago Law School, centers his analysis on demagogues. In his new book, “The Demagogue’s Playbook: The Battle for American Democracy From the Founders to Trump,” Posner takes us through the dangers of the charismatic, amoral, institution-destroying firebrands of American history to help us understand the specific threat that President Donald Trump poses to the republic. His conclusion: Trump’s threat is a dire one.

In Posner’s telling, demagogues have always been “crude, vulgar, and violent,” gathering popular support “through dishonesty, emotional manipulation, and the exploitation of social divisions.” In populist fashion, they blame political elites “for everything that has gone wrong” and try “to destroy institutions — legal, political, religious, social — and other sources of power that stand in their way.” Once in power, they seek to interfere in elections, undermine constraints on their activities and create division within the population to serve their “ultimate goal . . . personal power and glory.”

Posner reminds readers that when the founders were creating our constitutional order, they feared that the very nature of democracy might destroy their experiment in self-government. A charlatan seeking absolute power might lead the common people — ill-informed and easily manipulated — to achieve his goals. “There is never a democracy that did not commit suicide,” John Adams said. Alexander Hamilton feared politicians “paying an obsequious court to the people; commencing [as] demagogues, and ending tyrants.”

Fearing that demagogues would use democratic structures to manipulate the public to lift them into power, the founders built an elaborate set of impediments to a pure democracy: The Electoral College was originally intended to ensure that the president was not popularly elected; the Senate was first designed to have its members chosen by state legislators; the establishment of property requirements to vote or hold office was based on the rationale that economic and political independence were linked; and the separation of powers was meant to check a potential demagogue’s power. “For the Founders, nothing could be more obvious than that educated, experienced people should lead the government,” Posner observes. “The Founders created a ‘natural aristocracy’ of ‘virtue and talents,’ as Jefferson called it: rule by the elite.”

Since the nation’s founding, these bulwarks against the rise of a ruling demagogue have been dismantled, bit by bit, usually for very good reasons. The ideal of popular self-government always sat uneasily alongside the notion of a ruling natural aristocracy, particularly when the elite discredited themselves or abused their power. During the Jefferson administration, America’s western expansion eroded the property requirements to vote, and in the early republic states passed laws to ensure that Electoral College electors voted in accordance with state-sponsored popular presidential elections. Parties rose up and ensured elite vetting of candidates in backroom deals, but by the early 20th century some of their power was being delegated to the electorate through primary elections. In the Gilded Age, industrial interests exerted significant influence over party leaders and elected officials, paving the way for ratification of the 17th Amendment in 1913, which sought to break plutocratic control over the Senate by instituting the popular election of senators. Technological changes helped candidates communicate with the electorate through daily newspapers, radio, television, Twitter, while the population of voters expanded to include people of color, women and younger adults.

Demagogues were part of the political scene — Louisiana governor and senator Huey Long, Mississippi governor and senator Theodore Bilbo, Wisconsin Sen. Joseph McCarthy, to name a few — but only one, by Posner’s criteria, made it to the White House before 2016. Andrew Jackson was “a White Christian nationalist” with “a serious authoritarian and violent streak,” who had fought duels, beaten enslaved people, imprisoned a federal judge, slaughtered Native Americans and thrilled his base, Posner writes. In office he destroyed the independent civil service in favor of a spoils system that lasted for a half-century; crushed the independent central bank which, Posner argues, set the financial system back decades; and presided over a political system that swapped aristocrats for party bosses, who controlled the selection of presidential candidates and the spoils. The presidency was left diminished, and common people, instead of being empowered, found themselves under a new form of elite control.

Assessing the presidents who followed Jackson, Posner finds no demagogues until the arrival of our current chief executive. Trump gained the presidency, Posner writes, because conditions were propitious for such a candidate: The ruling elites had thoroughly discredited themselves through their mismanagement of the Iraq War, the eruption of the 2008 financial crisis, including the unpopular bailouts for those who caused it, and the long stagnation of wages for most Americans. Meanwhile, the final safeguard that might have stopped Trump — elite control of the party nomination process — had collapsed over the previous generation.

Posner asserts that his goal is to warn Americans of the danger Trump represents, not to put forth a prescription for addressing the republic’s vulnerabilities to demagogic rule. “We need to see Trump not merely as a poor choice for the presidency,” he concludes. “We need to see him as a political monstrosity who should be repudiated by the body politic, so that politicians who eye the presidency in the future will be deterred from using Trump’s ascendance as a model.”