Stephanie Kelton is the public face of Modern Monetary Theory. Photograph Courtesy Stephanie Kelton

Stephanie Kelton, a senior economic adviser to Bernie Sanders and a professor of economics and public policy at Stony Brook University, is popular in a way that economists, almost definitionally, are not. Filmmakers trail her with cameras; she goes on international speaking tours and once sold out a basketball arena in Italy. Kelton is the foremost evangelist of a fringe economic movement called Modern Monetary Theory, which, in part, argues that the government should pay for programs requiring big spending, such as the Green New Deal, by simply printing more money. This is a polarizing idea. This spring, Kelton spoke at the Wall Street Journal’s Future of Everything Festival, held in a converted warehouse in Tribeca, where earnest networkers milled around taking notes. On the dais, a Journal staffer introduced Kelton as an economist with an idea “that will either solve the world’s problems or send it into ruin!” She made a face, and then walked onstage.

I’d been stewing for a few months in the melange of blogs and YouTube videos and white papers that make up much of the M.M.T. world. Some intricacies lay beyond me—a hazy blur of literature about floating exchange rates and reserve currencies addled my brain. But the basic principle of M.M.T. is seductively simple: governments don’t have to budget like households, worrying about debt, because, unlike households, they can simply print their own money. So M.M.T. proposes that the constraint on government spending shouldn’t be debt but inflation: How much new money can you pump into the economy before prices rise?

Among a certain crowd—mostly online, and mostly on the left—M.M.T. has ignited a revolutionary fervor. On M.M.T. blogs and on M.M.T. Twitter, adherents imagine a world built on M.M.T. principles, in which the government provides guaranteed jobs, health care, and affordable college, and launches clean infrastructure projects to replace our crumbling highways, airports, and bridges. Kelton, who does at least five interviews per week, plus lectures, speaking gigs, and conferences, is, more than anyone, responsible for building M.M.T.’s digital army. She has written regular columns for Bloomberg; started the movement’s most influential blog, New Economic Perspectives; and is working on a book, “The Deficit Myth,” which will come out next year. “It’s pretty obvious she has become the most visible face of M.M.T.,” Randall Wray, one of the economists who first developed the theory, said. “She perfected the way to present these ideas to the public.”

An introduction to M.M.T. can provoke strong reactions. Maybe it’s not for you, and you find it ridiculous or even a little scary, or maybe it blows your mind—like your first time trying marmite or dropping acid. Kelton acts as a spirit guide. When she began her talk at the Wall Street Journal festival, I found a seat in the second row of the theatre, behind a woman in a white sweater with an eager, expressive face. She said her name was Ann. Ann had never heard of M.M.T.

Onstage, Kelton lamented, “There’s so much pressure on candidates to pay for everything. I don’t see anyone—I mean, I’ll just be honest, I don’t really see any Presidential candidates putting forward ambitious agendas and saying, ‘We’re not going to try to pay for any of this.’ ”

I saw Ann’s face register various states of shock. She mouthed, “What?!”

“It’s a tough sound bite,” the moderator, the Journal’s financial editor, Charles Forelle, said.

Kelton replied, “It is, right?” She went on, “What we’ve done to ourselves is to just leave trillions of dollars, literally, on the table, by not taking advantage of the fiscal space that we have, by running our economies below potential, by living below our means as a nation, year after year after year.”

The session wound down. “O.K.!” Forelle said. “Hands up if anybody’s got a question.” He peered out at the audience. “Oh! We’ve got a lot of questions!”

Kelton often hears the same concerns about M.M.T., and most are about inflation. How soon will we become Zimbabwe, which printed so many Zimbabwean dollars that inflation peaked, in 2008, at an annual rate of ninety sextillion per cent? Never, according to Kelton; under M.M.T., the focus is sustainable inflation, whereas fiscal traditionalists worry about the deficit and don’t consider inflation at all. Doesn’t M.M.T. then require accurate forecasting of inflation risk? Yes, and, Kelton conceded at the festival, the models aren’t perfect, “but we can do a pretty good job.” And, anyway, government spending, she believes, is responsible for just a small part of inflation.

Ann raised her hand but didn’t get called. When it was over, I caught up with her. “Did you hear me just say ‘Holy cow’?” she said. “It just seems like it’s exactly backward. But she did it so well that I can’t figure out why.”

I asked Ann whether she found Kelton convincing. “I mean, kind of!” she said. “I know what she said was brilliant; I just can’t believe her. She’s gotta be wrong.”

Kelton believes that, though M.M.T. is a new framework, it builds on old ideas found buried and forgotten in the work of foundational economists. The first person to begin assembling the pieces was a hedge-fund executive named Warren Mosler. A polymath with an iconoclastic streak, Mosler shopped around his ideas about money creation and the deficit in the early nineties, looking for allies and finding none. Working some connections, he eventually, in 1993, scored a meeting with Donald Rumsfeld, who was then working as an executive in the private sector. Rumsfeld said he could spare an hour at the Racquet Club of Chicago, in the steam room. Both men wore towels. When they emerged from the muggy haze, Mosler had won an ally.

Rumsfeld agreed to set up Mosler with a few economist friends. Most helpful was Art Laffer, the architect of supply-side economics, whose lifework, arguing for reducing taxes on the rich, recently earned him the Presidential Medal of Freedom from Donald Trump. Laffer had popularized the contentious notion that reducing taxes can actually increase tax revenues. Mosler, by contrast, wanted to prove that tax revenues were irrelevant to government spending. But Laffer helped Mosler workshop his ideas and directed him to a group of post-Keynesian economists who ran a boisterous Listserv—a Reddit for the dial-up age. Mosler logged on and found the economists who became M.M.T.’s founding thinkers.

Today, Mosler lives in St. Croix, a U.S. territory where he can avoid paying ninety per cent of his federal income tax. (“This is an actual U.S., federally sponsored program,” he told me. “I’m doing my patriotic duty.”) Mosler estimated that he has contributed about three million dollars to the M.M.T. movement in the course of a couple of decades, and, “if anything, I get kind of defensive about not having spent more.” The money has subsidized academic posts, conferences, and scholarships and has helped turn institutions like the Levy Institute, at Bard College, and the University of Missouri–Kansas City into fertile grounds for M.M.T. thought.

Kelton first encountered M.M.T. in the mid nineteen-nineties, when, as a graduate student at Cambridge University, she came across Mosler’s online agitating. Kelton applied for a fellowship at the Levy Institute, where many of the early M.M.T. thinkers had gathered. There, in 1998, she authored one of M.M.T.’s foundational texts, a paper titled “Can Taxes and Bonds Finance Government Spending?” The paper concludes that taxes don’t actually pay for anything—that the federal government spends first, then taxes some of that money back later. Kelton went on to get her Ph.D. from the New School, then was hired by U.M.K.C. In 2013, she became the chair of its economics department. Soon, she became the preferred interlocutor of hedge-fund managers and politicians who had questions about M.M.T. She held meetings with members of Congress. Larry Summers, who had recently stepped down as the director of the National Economic Council under Barack Obama, solicited M.M.T. literature.