Over the last few days, President Trump has signaled a major shake-up at the Department of Homeland Security. Secretary Kirstjen Nielsen was asked to step down; other top officials may soon follow. White House sources say it’s part of a plan to get tougher on immigration.

One key person behind that push: White House aide Stephen Miller.

Miller helped craft some of the president’s most controversial immigration policies. He helped draft an executive order banning travelers from seven predominantly Muslim countries from entering the United States. More recently, he helped craft the family separation policy, designed to discourage families from seeking asylum at the U.S.-Mexico border.

Of course, the president’s hard-line position on immigration didn’t start with Miller. The day Trump launched his presidential campaign, he accused Mexico of sending people “that have lots of problems,” to the United States. “They’re bringing drugs,” Trump said. “They’re bringing crime. They’re rapists.”

Since partnering with Trump’s agenda, Miller has advocated for policies that would restrict immigration.

He has explained this as a broader push for law and order. “Right now, as we speak, there is a surge of illegal immigration heading towards our country that presents a national crisis now,” Miller said on CNN, while defending the president’s government shutdown in January. “This president took an oath like every lawmaker in Congress to defend the citizens of this country. How many more innocent people have to die in pursuit of an open-borders agenda?”

But his critics see something darker: a fight for a particular kind of American identity, framed around race.

Miller, the son of Jewish Democrats, grew up in white, affluent, liberal Santa Monica, Calif. Jason Islas, a liberal activist and a childhood friend of Miller’s, told the Atlantic that it was the type of community that “conspicuously celebrate[d] diversity in very self-congratulatory ways” while ignoring some deeper race issues.

Even in high school, Miller pushed back against these ideas, often arguing that he was doing so to promote free speech. Conversations about diversity and multiculturalism were prevalent at Santa Monica High School.

Adrian Karima, an Iranian immigrant who took an advanced-placement government class with Miller, said Miller often accused the teacher trying to indoctrinate students with liberal politics. “He saw it as being outnumbered in Santa Monica,” she told CNN, and argued for “trying to preserve any idea of Americanism that he felt, particularly by pushing English-speaking first.”

That emphasis on race is echoed by others. “Shortly before they started high school, Islas recalled, Miller informed him that they couldn’t be friends anymore, citing Islas’s ‘Latino heritage’ as one of several reasons,” the Atlantic reported.

Miller continued to provoke and nurture his conservatism at Duke University. As a student, Miller invited conservative activist David Horowitz to campus and wrote a controversial college newspaper column in which he argued, among other things, that multiculturalism threatens American identity.

After college, Miller moved to Washington, D.C., to work for members of Congress. He began his career with then-Rep. Michele Bachmann (R-Minn.) and eventually moved to the office of then-Sen. Jeff Sessions (R-Ala.). In 2016, Miller joined Trump’s presidential campaign as a senior policy adviser.

In that role, he has secured an “iron grip on Trump’s immigration policies,” Politico writes, and has advocated for “radically tighter borders.” He has praised the president’s reading of a poem at campaign rallies comparing Syrian refugees to poisonous snakes. He supports the president’s border wall and has defended the shutdown that accompanied it. He has pushed to slash the number of refugees the United States accepts and to make it harder for undocumented immigrants to obtain legal status.

Miller rejects the idea that he is shaping the president’s immigration policy. He says he is just carrying out what Trump wants. “My job is simple,” he told the Atlantic. “The president has made clear what he wants to accomplish, and I’ll do the best I can to help that happen.”

But his critics don’t see it that way. Miller, they argue, is advocating for policies that would benefit white Americans and keep others out. “Stephen Miller is a white nationalist,” Rep. Ilhan Omar (D-Minn.) tweeted Monday. “The fact that he still has influence on policy and political appointments is an outrage.”

Richard Painter, a White House ethics lawyer during President George W. Bush’s administration, called Miller “the most notorious racist in the Trump White House.”

Trump has been repeatedly accused of emboldening white nationalism with his divisive rhetoric, unwillingness to speak out aggressively against racism and immigration policies. Increasingly, it appears he is relying on Miller to shape those approaches.