In 1656, in the aftermath of the English Civil War, the political theorist James Harrington outlined an ideal form of government. In The Commonwealth of Oceana, he imagined a country that, like his own, had overthrown its monarch and was now governed by its own people in their common interest. Harrington recognized that it would be hard to divide power equally throughout this new society. In “class warfare” constitutions—those that split power between the nobility and the people—one group inevitably came to dominate the other, “as the people did the nobility in Athens, and the nobility the people in Rome.” The result was tyranny or anarchy.

Harrington’s major insight was that popular government could only be stable if the people were political equals, which meant they must also be economic equals. Political power, he saw, was precisely correlated with wealth: When property is concentrated among the few, landowners can wield undue influence and distort the balance of power. A true commonwealth, Harrington argued, required that “the whole people be landlords” so that no one group could “overbalance” the rest. The people of his imaginary Oceana are each granted a parcel of land, thus breaking up large fortunes and estates.

THE CRISIS OF THE MIDDLE-CLASS CONSTITUTION by Ganesh Sitaraman Knopf, 432 pp., $28.00

Harrington was not the first to envision a society ruled by neither rich nor poor. Donato Giannotti, a leader of the Florentine Republic in the sixteenth century, saw promise in the mediocri, his city’s new and expanding middle class. If the mediocri could grow larger than rich and poor put together, he speculated, they could hold the balance of power and ensure a degree of stability. Aristotle saw similar promise in the “middling” stratum he observed in the fourth century B.C. in Athens. Because they shared neither the overwhelming burdens of the extremely poor, nor the all-consuming vices of the very rich, they were uniquely positioned to mediate between the two extremes. But Aristotle did not make more of this line of thought, because the middle class was not yet large enough to form a majority. That would have to wait for the founding of America.

In his new book, The Crisis of the Middle-Class Constitution, Ganesh Sitaraman proposes that America in the eighteenth century represented the first instance in history of a country with a middle class large enough to permit the realization of a commonwealth. Two-thirds of white Americans owned land, compared to only one-fifth of the population in England. “Conditions approaching the slums of London,” Sitaraman observes, “were unknown in America.” White colonists also enjoyed remarkable equality of wealth: The top 1 percent of Americans possessed 8.5 percent of total income in 1774, compared to 19.3 percent in 2012. (Sitaraman deals only glancingly with slavery, acknowledging that “economic equality and economic opportunity did not extend to everyone.”) Even six decades later, Alexis de Tocqueville found a country where the people were “more equal in their fortunes ... than they are in any country in the world and than they have been in any century of which history keeps a memory.”

Not only were American statesmen of the era keenly aware of the uniqueness of their economic situation, but they were also guided by Harrington’s vision of a republic of equals. Harrington, John Adams wrote, “has shown that power always follows property.” The only way to preserve “equal liberty and public virtue” in a republic, Adams reasoned, would be to “make the acquisition of land easy to every member of society.” If a majority of people owned land, they would also hold the balance of political power. The Founding Fathers, however, were also aware of the inverse: that the republic could not withstand drastic inequality. Sitaraman begins his book, in fact, by identifying the collapse of the middle class in our time as the greatest single threat to America’s constitutional government. As the middle class has declined—from 61 percent of the population in 1971 to less than 50 percent in 2015—the power of the superrich has soared. The 20 richest Americans are now wealthier than half of the country’s entire population combined. And economic power translates directly into political power: Between 1983 and 2012, corporations’ expenditure on lobbying soared from $200 million to $3.3 billion. Amid such extreme imbalances, Sitaraman argues, we are now faced with a choice—to accept our current state of oligarchy, or to attempt to restore the equality on which the republic was founded.