The Southern Poverty Law Center released a cache of hundreds of emails Tuesday leaked by a former Breitbart writer who corresponded with Miller between 2015 and 2016.
Several prominent Democrats have called for White House senior adviser Stephen Miller to resign following the release of emails in which he purportedly linked to white nationalist websites.
The Southern Poverty Law Centerreleased a cache of hundreds of emails Tuesday leaked by former Breitbart writer Katie McHugh, who corresponded with Miller between 2015 and 2016. In the correspondence, Miller urged Breitbart's increased coverage of crimes by Hispanic and nonwhite people.
NBC News has not seen the emails or been able to confirm their authenticity.
Miller was appointed senior policy adviser to the Trump campaign in January 2016 and later joined President Donald Trump's transition team. He currently advises the president on immigration policy and is credited with spearheading Trump's hard-line efforts on border security.
Rep. Alexandria Ocasio-Cortez, D-N.Y., tweeted a petition demanding Miller's resignation.
"Stephen Miller must resign. Now," Ocasio-Cortez tweeted. "Each day we allow a white nationalist to be in charge of US immigration policy is a day where thousands of children & families lives are in danger."
Other Democratic politicians made similar statements. Rep. Ilhan Omar, D-Minn., said Miller needs to step down. Democratic presidential candidate Julián Castro called Miller and Trump "a shame to our nation." Rep. Earl Blumenauer, D-Ore., tweeted that Trump should fire Miller immediately.
Conservative politicians did not echo those calls, but Tiana Lowe, a commentator for The Washington Examiner, a newspaper that leans conservative, called for Trump to fire Miller.
The emails appear to include a variety of correspondence on immigration, with McHugh and Miller exchanging links to a variety of far-right websites, including two that the SPLC has called hate groups.
Asked for comment on the SPLC's report, a White House official said: "This is clearly a form of anti-Semitism to levy these attacks against a Jewish staffer."
In the emails, Miller appeared to recommend links to white supremacist websites VDARE and American Renaissance, which have both heavily trafficked in the "Great Replacement" conspiracy theory. The "Great Replacement" or "White Genocide" theory posits that white people are intentionally being "replaced" in America by other races.
McHugh told Michael Edison Hayden, a senior investigative reporter at the SPLC, that she was "struck" by the way Miller referred to American Renaissance, a white supremacist think tank, by its nickname, AmRen.
Hayden wrote that the SPLC "was unable to find any examples of Miller writing sympathetically or even in neutral tones about any person who is nonwhite or foreign-born."
Miller consistently pushed McHugh to cover stories in a certain way, often highlighting angles on race. After an alt-right mass shooter, Chris Harper-Mercer, killed nine people at the Umpqua Community College in Oregon in 2015, Miller asked Breitbart to highlight his race and make it the most prominent part of the story.
"He is described as 'mixed race' and born in England. Any chance of piecing that profile together more, or will it all be covered up?" according to one of the emails the law center said Miller sent to McHugh.
McHugh was fired from Breitbart in 2017 for tweets she later admitted were racist. McHugh has since disavowed her career at Breitbart and its far-right politics.
"You have to own up to what you did and then forcefully reject this and explain to people and tell your story and say, 'Get out while you can,'" she told BuzzFeed News in May.
Miller is frequently credited with some of the Trump administration's most extreme immigration policies. Miller and former Attorney General Jeff Sessions were the primary proponents ofTrump's child separation policy, which separated migrant children from parents seeking to enter the U.S. at the border with Mexico.
Hayden told NBC News that McHugh sent him the emails in June, in part because of his previous reporting on extremists and white nationalists.
He said the emails struck a tone that was similar to that used by other extremists.
"It's the sort of intellectual white nationalism that's very familiar to people covering this stuff," Hayden said.
Hayden said the SPLC is set to publish more emails from Miller in the coming days, specifically ones in which Miller targeted Republican opponents of then-candidate Trump in 2015 and 2016. Hayden said McHugh had only met Miller in person once but still sent her almost 1,000 emails.
"And we only have the emails that start in 2015," Hayden said. "It shows how the degree to which laundering material was going on between Miller and conservative media."