Supporters of Democratic presidential candidate Hillary Clinton cheer as they listen to her speak in Louisville on May 10. For Clinton, whether she is competing against Bernie Sanders or Donald Trump, one concern is much the same. They are outsider candidates riding a wave of populist excitement, while she is viewed as a traditional, establishment choice. As a result, her campaign sometimes just looks a little less exciting. (AP Photo/Patrick Semansky)

Populism is back, as a look at this year’s presidential campaign tells us. Whether they see it as good or bad, many observers (including several here at The Monkey Cage) have drawn parallels between Donald Trump and Bernie Sanders, suggesting that their supporters share similar concerns.

And pundits are arguing that Hillary Clinton — now the Democratic Party’s presumed nominee — must harness these populist energies to succeed in November’s election. But is populism necessarily exclusionary and right wing, as you might guess from Trump’s call to ban Muslims from entering the U.S. and to build a border wall to keep Mexicans out?

After all, the party platform ratified at the 1892 founding convention of the People’s Party of America, which gave populism its name, condemns an immigration policy that “opens our ports to the pauper and criminal classes of the world and crowds out our wage-earners; and we denounce the present ineffective laws against contract labor, and demand the further restriction of undesirable emigration.”

And the populism of both George Wallace and the tea party movement rely on racial resentment and anti-immigrant views. Even Sanders can seem indifferent to outsiders. When he decries the trade policies that allow U.S. corporations to “pay slave wages in Mexico or China,” he usually implies that the solution is to bring U.S. jobs back rather than boost wages abroad.

In Europe, Greece’s Syriza and Spain’s Podemos have won with left-wing populism – based on the theories of the same little-known political theorist

But it’s not always so. In Europe, populist parties like Spain’s Podemos and Greece’s Syriza have won by campaigning against austerity policies and in favor of rebuilding the welfare state while explicitly opposing xenophobia. Instead of directing populist anger at immigrants and other outsiders, they direct it toward the political and economic elites whose policies create inequality. Podemos’s current platform details policies against almost every conceivable form of discrimination, even proposing a “Celiac law” that would end discrimination against people who need gluten-free food.

Syriza has governed Greece since January 2015. Podemos was only founded in March 2014 and has already become the second-largest political party in Spain.

And they have something unusual in common: Both Podemos and Syriza are led by politicians whose approaches are explicitly drawn from radical democratic theorist Ernesto Laclau’s 2005 book “On Populist Reason.” What do these European populists find in Laclau’s theory? And what can it tell us about the prospects for left-wing populism in the U.S.?

Laclau draws much of his argument from difficult theorists like Jacques Derrida and Jacques Lacan, but his central insight is simple: Engaging in politics can give people new identities. A great deal of political science assumes that citizens come to politics with political preferences already in place, and treats politics as conflict between pre-existing interest or identity groups.

Laclau argues that this underestimates the power of politics to change how we understand who we are. Populism does this by linking demands together so that they form what Laclau calls an “empty signifier” — a popular demand whose policy content is less important than the opportunity it provides to identify with “the people” as a whole.

Laclau’s idea of the empty signifier helps explain several features of populism that would otherwise be puzzling. He emphasizes the importance of identity over policy — which makes it easier to understand how someone could be enthusiastic about both Trump and Sanders when the candidates support very different policies. Podemos and Syriza, therefore, believe that if they offer the right vision of “the people,” voters will be enthusiastic about their platforms.

That’s also why there’s no contradiction between Trump’s populism and the relative wealth of his supporters. Populism isn’t the political expression of a pre-existing working class. Rather, it’s how individuals use a grievance to identify themselves as the authentic embodiment of “the people” — unlike those other people, the group they are blaming for that grievance.

You can see this in the tea party. Research by Vanessa Williamson, Theda Skocpol and John Coggin shows that tea party activists typically identify as “workers” whose virtuous labor makes them part of the American people, who therefore deserve government support like Medicare. That was true even though a third of them were “students, unemployed people, or retirees” and even though most tea partiers had higher than average incomes. Williamson, Skocpol and Coggin report that activists identified “young people and unauthorized immigrants” as “non-workers” who “who may try to freeload at the expense of hardworking American taxpayers.”

But if Laclau is right, a populist identity is an empty signifier, meaningful in itself. Depending on how it’s constructed, that identity could direct dissatisfaction and anger in an entirely different direction — for instance, targeting elites rather than immigrants, bankers rather than terrorists. Populism could offer a vision of “the people” that directs antagonism away from the marginalized and toward the powerful.

But there’s a flaw in this vision of populism

Is it true? Populist identities may not be so easily redirected. Despite his left-wing credentials, Laclau’s view of politics is close to the tea party’s in important respects. Essentially, he argues that, for a set of demands to become an empty signifier that can stand for the people as a whole, the demands — and accordingly the people within the movement that makes those demands — need to be equivalent to each other. A win for one demand needs to be seen as a win for all of them. Only then can populists stand as one against their antagonists.

And in order for people to be equal, they need to see themselves as internally homogeneous. Only when everyone is a “worker” in the same way can the empty signifier create a coherent political identity.

But there’s a problem with this kind of populism, which is oddly unnoticed by Laclau. Some groups are durably identified as different, like immigrants or African Americans. When they demand to be treated as equals, many populists see that as special pleading. To want to be different and equal seems unfair to the populist mind-set. The result, as Michael Tesler and others have noted here in The Monkey Cage, is racial resentment.

The problem is that populism conflates equality with homogeneity. Populism can minimize differences in the name of unity, but it doesn’t make those differences go away. For example, if pointing out that white workers and black workers face significantly different employment conditions disrupts the unity of “the people,” then populist politics will leave African Americans behind.

That’s what Syriza’s president Alexis Tspiras, the prime minister of Greece, recognized when, in defending Greece’s acceptance of refugees, he said, “Even populism and trying to win votes must have some limits.” Similarly, Podemos has found practical limits to Laclau’s view when it confronts issues important to national minorities, such as the demand for Catalonian independence.

Here’s what can Democrats learn from Laclau and his left-wing populist followers. If they want to harness populism to promote inclusive policies, they could offer an image of the American people struggling against an elite – and drawing strength from differences among one another, rather than being threatened by them.

Can that be an effective electoral strategy? We may not need to wait until November for further evidence. Spain has just called new elections for June 26.

Benjamin L. McKean is an assistant professor of political science at Ohio State University.