It should be easy for Bernie Sanders to get to the left of Hillary Clinton. The Clintons have long dabbled in centrist Democratic Leadership Council politics, while Sanders is an avowed socialist, albeit a small-d democratic one.

As such, it's no surprise that Friends of the Earth, a major environmental group, has endorsed Sanders for president in response to Clinton's dithering over the Keystone XL pipeline. Leaders of large labor unions like the AFL-CIO admit that Sanders is generating more enthusiasm from the rank and file. Sanders is polling competitively in New Hampshire and drawing huge crowds elsewhere, all while raising $15 million from small donors.

Yet it was Sanders the socialist who was effectively heckled by Black Lives Matter activists at the Netroots Nation conference last month. Clinton didn't attend the progressive confab, but she picked up on Sanders' unease, and has since incorporated the racial-justice phrase into her speeches.

After Netroots, Sanders again faced a great deal of pushback from the left when he told Ezra Klein that he wasn't a fan of open borders. "You know what youth unemployment is in the United States of America today?" Sanders asked incredulously. "If you're a white high school graduate, it's 33 percent, Hispanic 36 percent, African American 51 percent. You think we should open the borders and bring in a lot of low-wage workers, or do you think maybe we should try to get jobs for those kids?"

There was a time when this wouldn't have been such a heretical viewpoint on the left. But that time has come and gone. These days, it's hard to find a liberal this side of Mickey Kaus who thinks restricting immigration for the benefit of American workers is something progressives should contemplate. Some went so far as to argue Sanders' opposition to open borders was "ugly" and "wrongheaded," since "no single policy the United States could adopt" would "do more good for more people." It didn't take long for Sanders to backtrack slightly, telling Univison's Jorge Ramos he'd consider opening borders between the U.S., Canada, and Mexico.

Kaus, our lonely liberal immigration skeptic, asked what happened to Sanders' concern about American wages: "Do unskilled Mexicans have some magical properties that suspend supply and demand that unskilled immigrants from other countries lack?"

Sanders defended his immigration views when speaking to the U.S. Hispanic Chamber of Commerce. He acknowledged that his history representing a 95 percent white state may make minorities worry he is out of touch with their concerns. But that's only part of Sanders' problem.

Bernie Sanders is an old-school progressive who believes most of the country's problems can be traced to class and economics. Meanwhile, contemporary progressivism is more committed to multiculturalism and the idea that America's biggest injustices remain inextricably tied to race.

On a lot of substantive policy issues, this is a distinction without a difference. Most liberals recognize there is a strong relationship between economics and structural racism. Sanders favors most of the same policies his multicultural critics do and is even, on balance, pretty supportive of high levels of immigration.

But there are important differences rhetorically and in terms of how you conceptualize the government's obligations. You don't have to believe Sanders has anything in common with Joseph Stalin's politics to recognize that he is also talking about "socialism in one country."

Sanders favors a robust welfare state and wants the government to mandate generous wages and working conditions. But he wants those things for Americans, not necessarily all the people living all across the globe whose standard of living could theoretically be improved by residing in America instead. (Rand Paul gets similar grief when he occasionally advocates libertarianism in one country.)

This puts Sanders out of step with much of his party. It also gives Clinton an opening to Sanders' left, at least rhetorically, on some racial issues, which could limit his following to college-educated liberal whites. This is crucial, because the ability to reach beyond these voters and win over minorities was the difference between Barack Obama and Howard Dean.

Unless Sanders can, at 73, update his socialism to fit in with the priorities and demands of today's left, Clinton can keep him contained — and Joe Biden can keep his faint presidential hopes alive.