Senator Bernie Sanders, whose 2016 presidential campaign legitimized the term “socialist” and made single-payer health care, free college tuition, and a $15 minimum wage credible issues for the Democratic Party, faces his most daunting challenge to date. A new generation of progressives is seeking to make immigration a firm dividing line between the two political parties, putting Sanders in a tough position.

For the better part of the 2018 election cycle, the Democratic Party, which Sanders views as his best vehicle to capture the presidency, has struggled to develop a singular message on immigration.

At various points, the party has attempted to paint the coming midterms as not only a referendum on President Donald Trump, but also on such wedge issues as tax cutsgun control, health care, and abortion, among others. Each of these topics, while appealing to segments of the Democratic constituency, have failed to energize, let alone impassion, the party’s increasingly left-leaning base.

That is, however, until immigration appeared on the national radar in the form of the Trump administration’s “zero tolerance” policy on illegal border crossings.

That initiative, which sought to limit illegal immigration by charging with unlawful entry any individual who crossed the U.S.-Mexico border without authorization, triggered a backlash from Democrats and liberal activists. The main point of contention was over allegations that the policy resulted in the separation of migrant children from their parents due to federal regulations that prevented youths from being housed in the same detention facilities as adults.

Sensing an opportunity to bolster their electoral chances, Democrats flocked to the border in an attempt to seize the moral high ground. Yet few noted that their own party’s congressional leadership was partially to blame, having rebuffed overtures by the president to find a legislative solution to the problem.

The Democrats’ emotional appeals to keep “families together” quickly devolved into support for unmitigated migration and the abolition of U.S. Immigration and Customs Enforcement (ICE), the federal agency responsible for enforcing immigration laws.

Previously a fringe issue associated with the far left, the movement to shutter ICE is now a rallying cry for the Democratic Party heading into the midterm elections. Not only has it picked up steam with candidates up and down the ballot, it’s also garnered the backing of potential 2020 Democratic presidential contenders like Senator Kirsten Gillibrand, Mayor Bill de Blasio, and Senator Elizabeth Warren.

All of this has put Sanders, the left’s torchbearer who nearly toppled the ill-fated Hillary Clinton during the 2016 Democratic presidential primaries, in a precarious position. Sanders, long wary of open immigration on economic grounds, now finds himself increasingly at odds with a left clamoring for open borders.

This was on display during an appearance on CNN’s “State of the Union” with Jake Tapper, where Sanders was asked whether he supported abolishing ICE. He demurred, choosing instead to call on the president and Congress to “sit down” and deal with “this serious issue” in a “rational way.”

Sanders’ refusal to call for ICE’s abolition triggered outrage from progressive activists, like Jeremy Scahill—co-founder of The Intercept—who castigated the senator for having chosen to stand “on the wrong side of history.”

Liberal publications were no less forgiving. The New Republic offered a particularly fiery denouncement that accused Sanders, a self-described Democratic socialist, of being neither a “doctrinaire leftist” nor a true “revolutionary.”

“It’s certainly true that Sanders is to the left of most Democrats,” writer Sarah Jones contended. “But contrary to how he’s often portrayed in the media, he is not a doctrinaire leftist. His principal benefit to the left has been to mainstream certain beliefs—namely, that access to health care, education, and living wages are rights, not luxuries. But Sanders is not a revolutionary.”

In response to mounting criticism, Sanders attempted to clarify his stance by pointing to the fact that he was one of the only House members to vote against ICE’s creation in 2002. In the hope of further appeasing the left, Sanders declared his support for abolishing “the cruel, dysfunctional immigration system we have today” in favor of “comprehensive immigration reform.” Perhaps tellingly, however, he did not commit himself to the dissolution of ICE, only saying that the agency was in need of “restructuring.”

Sanders’ demurral on this matter and the response it provoked underscore how far the senator has diverged from liberal orthodoxy on the issue of immigration.

Throughout his tenure in Congress, Sanders has been generally supportive of creating a pathway to citizenship for the 11 million illegal immigrants currently residing in the United States. He has also advocated for extending the Obama-era Deferred Action for Childhood Arrivals (DACA) program, which granted legal protections to nearly 800,000 illegal immigrants brought to the country as children.

While such sentiments are very much in line with the majority of the Democratic Party, Sanders’ views on migration—both lawful and unlawful—as well as border control are not.

In 2007, Sanders joined with conservatives in the Senate to defeat a comprehensive immigration reform measure championed by the late Ted Kennedy and then-president George W. Bush. He opposed the legislation on the grounds that it would create a new visa program to “bring millions of ‘guest workers’ into this country who are prepared to work for lower wages than American workers.”

In the past, Sanders has opposed expanding the cap on the number of H-1B visas, granted to guest workers in highly specialized fields of occupation, because this would serve to “depress wages.”

Sanders has also shown hostility to the notion of open borders, which might explain his unwillingness to endorse abolishing ICE.

In 2015, shortly after announcing his presidential bid, the senator conducted an interview where he was asked if he believed global poverty could be solved by “sharply raising the level of immigration,” perhaps even to the point of “open borders.” Sanders lambasted the idea, claiming it was a “right-wing proposal.”

“It would make everybody in America poorer—you’re doing away with the concept of a nation-state, and I don’t think there’s any country in the world that believes in that,” he said, adding: “What right-wing people in this country would love is an open-border policy. Bring in all kinds of people, work for $2 or $3 an hour, that would be great for them.”

Sanders’ view on the adverse economic effects of immigration stands in stark contrast to the Democratic Party’s adherence to identity politics, driven in part by the belief that “demographics are destiny.”

His contention that open borders destroy the “concept of a nation-state” is far closer to President Trump’s assertion that “a country without borders isn’t a country” than to Alexandria Ocasio-Cortez’s argument that illegal migrants have a “right of passage” to enter the U.S. freely.

Sanders’ stance on immigration again reared its head during the 2016 Democratic primaries, when Hillary Clinton accused him of being closer to Trump on immigration than herself. Sanders, for his part, was able to diffuse the controversy by otherwise being far to the left of Clinton and the other Democrats vying for the party’s nomination, thereby leaving progressives with no other alternative.

As The New Republic noted, Sanders’ preeminent contribution to the current political landscape has been to shift the Overton Window on issues from universal health care to free college tuition. In 2020, should Sanders run, he is not likely to have the luxury of competing against candidates to his right in what is expected to be a wide and ideologically diverse field.

In fact, given how far some Democrats have moved to the left, Sanders could even be viewed as something of a moderate in the 2020 crowd—at least when compared to the likes of Bill de Blasio and Elizabeth Warren.

Haris Alic is a social media writer at the Washington Free Beacon.