Former Democratic presidential nominee Hillary Clinton said Tuesday that White House senior adviser Stephen Miller, whom progressives have blamed for the president's hard-line immigration agenda should be removed from his position.

"Every day Stephen Miller remains in the White House is an emergency," Clinton tweeted before promoting a letter calling for Miller's removal, signed by organizations like the Southern Poverty Law Center (SPLC) and NAACP.

The letter was produced by The Leadership Conference on Civil and Human Rights, a nonprofit coalition of "more than 200 national organizations to promote and protect the civil and human rights of all persons in the United States," according to its website.

The letter accused Miller of supporting white supremacy and stoking bigotry during his career.

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"Supporters of white supremacists and neo-Nazis should not be allowed to serve at any level of government, let alone in the White House," it read. "Stephen Miller has stoked bigotry, hate, and division with his extreme political rhetoric and policies throughout his career. The recent exposure of his deep-seated racism provides further proof that he is unfit to serve and should immediately leave his post."

The SPLC, a liberal nonprofit known for accusing conservatives of "hate," claimed last week that Miller sent emails "promoted white nationalist literature and racist propaganda" to conservative news site Breitbart in 2015 and 2016 when he was working for then-Sen. Jeff Sessions, R-Ala.

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"Every day Hillary Clinton was in office WAS an actual emergency," White House Press Secretary Stephanie Grisham said in a statement to Fox News. "Lest we forget her policies helped create a massacre in Benghazi, Libya, a humanitarian disaster in Syria, and the rise of ISIS in Iraq. Stephen Miller is dedicated to this country and I am proud to work alongside him every single day with the goal of making our nation even greater. He is a friend and colleague, and we are lucky to have him in the White House."

Following the publication of the SPLC's report, Rep. Alexandria Ocasio-Cortez, D-N.Y., called for Miller's ouster. "Each day we allow a white nationalist to be in charge of US immigration policy is a day where thousands of children & families [sic] lives are in danger," she said. "This year alone, under Miller’s direction, the US has put almost 70,000 children in custody."

After Rep. Ilhan Omar, D-Minn., a fellow "Squad" member, criticized Miller, Trump quoted someone accusing her of targeting "Jews."

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The SPLC report also claimed that Miller shared white nationalist websites, a “white genocide”-themed novel, xenophobic conspiracy theories and eugenics-era immigration laws that Adolf Hitler lauded in “Mein Kampf.” The report concludes the emails show that Miller used these ideologies to “as an architect” for Trump administration immigration policies, included the travel ban, zero-tolerance police, which resulted in the separation of children at the border, and undocumented immigrant arrest quotas.

Fox News' Joseph A. Wulfsohn and Danielle Wallace contributed to this report.