Maverick candidates Bernie Sanders and Donald Trump are carrying on a long-standing American tradition. Here, a brief primer on populism.

What is populism?
Broadly speaking, it's the belief that the will of ordinary citizens should prevail over that of a privileged elite. Throughout American history, movements based on anti-elitism have repeatedly sprung up on both the left and right, often stoked by charismatic firebrands who harnessed the resentment of marginalized people. Today, both the Democratic and Republican parties have been splintered by populist movements. Bernie Sanders, a self-described "democratic socialist" who rails against income inequality and the billionaire class, is mounting a serious challenge to Democratic frontrunner Hillary Clinton. The Republican race, meanwhile, has been roiled by the right-wing populist campaign of real estate mogul Donald Trump, who vows to deport all 11.5 million illegal immigrants and build a massive wall at the Mexican border. Neither Sanders' nor Trump's message is really new. Sanders has picked up where the late-19th-century Populist Party left off, and as former Texas Gov. Rick Perry recently observed, "Donald Trump is the modern-day incarnation of the Know-Nothing movement."

Who were the Know-Nothings?
They were a xenophobic political movement that arose in the 1840s, in reaction to a huge influx of Irish Catholic and German immigrants. Native-born Protestants saw these immigrants as job-stealing threats to America's cultural and religious identity. The Know-Nothings began as secret societies — asked about their ties to these groups, members were instructed to say they "knew nothing." But they came out of the closet in 1855 to form the American Party, demanding immigration restrictions and a 21-year residency requirement for citizenship. In 1856, the Know-Nothings chose former President Millard Fillmore as their nominee, and he won 21.6 percent of the vote. Later, a rift between anti-slavery and pro-slavery factions fatally splintered their movement, but nativism has flared anew with every successive wave of immigration.

What about left-wing populism?
The first movement of this kind was started in the 1880s, by farmers who were suffering because of plummeting cotton prices in the South and a drought in the Great Plains. As farmers sank deeper into debt, their simmering resentments of Eastern elites were ignited, especially by bankers charging exorbitant lending rates and railroad barons charging high prices. The farmers, labor unions, and their sympathizers formed what they officially called the People's Party but was commonly known as the Populists. The Populists felt "squeezed by the unfettered capitalism of the Gilded Age," says Rutgers University historian David Greenberg. The Populists wanted to nationalize railroads, break up big trusts, and get rid of the gold standard, which restricted the money supply. They also advocated an eight-hour workday, women's suffrage, and a progressive income tax. In 1892 Populist presidential candidate James B. Weaver won 8.5 percent of the vote. But it was downhill from there.

What happened?
The Populists split into two factions: "fusionists," who thought the party should merge with the Democrats, and Populists, who preferred independence. The fusionists prevailed, rallying behind 1896 Democratic presidential candidate William Jennings Bryan, whose convention address decrying the gold standard — "You shall not crucify mankind upon a cross of gold" — remains one of American history's most famous speeches. He lost the election to Republican William McKinley, however — and went on to lose two more. But Bryan left a lasting Populist legacy. He "was the first leader of a major party to argue for permanently expanding the power of the federal government to serve the welfare of ordinary Americans," says biographer Michael Kazin.

What became of the Populists?
Many of their core ideas were absorbed by the Democratic Party and became the foundation of Franklin Delano Roosevelt's New Deal. More radical versions also sprang up during the Depression, which saw the meteoric rise of Huey P. Long. But during the Cold War, anti-elitism "began to slip its liberal moorings," Kazin says. After controlling federal power for a generation, liberals were the elite — and populism took a hard right turn. The anti-Communist crusade of Sen. Joseph McCarthy trained its rhetoric mostly on left-leaning academics, Ivy League–educated officials, and Hollywood actors and producers. In the 1960s, segregationist Alabama Gov. George Wallace played the working-class hero, snarling at "pointy-headed bureaucrats" and liberals. His third-party presidential bid in 1968 drew 13.6 percent of the vote. Conservatism has had a strain of anti-elitist populism ever since, most recently and effectively in the Tea Party.

Why has populism returned?
The 2008 financial crisis sparked an explosion of anger against Wall Street and Washington. In Sanders, left-wing populism comes full circle — his stump speeches would have played well in the 1890s. Trump has taken the old nativist message, added a big dose of narcissism, and turned his movement into a cult of personality. But throughout history, populists from left to right have had something in common besides anti-elitism. While they often influence mainstream parties, they don't win national elections. The majority of voters reject "their Us versus Them mentality," says columnist David Brooks, making the history of populism "generally a history of defeat."

The Kingfish
"Every man a king." That was the slogan of Huey Pierce Long Jr., the Louisiana governor and senator of the 1930s who was arguably the most flamboyant populist in American history. The pugnacious country boy called himself The Kingfish, and was a sworn enemy of oligarchs and corporate interests and boasted of buying legislators "like sacks of potatoes." In the depths of the Depression, Long's "Share Our Wealth" plan called for the federal government to confiscate the fortunes of anyone with more than $8 million in wealth to provide a $5,000 annual income ($71,450 in 2015 dollars) and health care for all American families. As governor, Long built thousands of miles of roads and improved education, but was also notoriously corrupt and dictatorial. Franklin Roosevelt called him one of the most dangerous men in America, with good reason: The Kingfish was widely considered a viable dark-horse candidate to defeat FDR in 1936. But he was assassinated by the relative of a political foe. Long's last words: "I wonder why he shot me."