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WASHINGTON — California Democratic Sen. Kamala Harris on Monday called for the resignation of Department of Homeland Security Secretary Kirstjen Nielsen as a result of immigration policy that has resulted in family separations at the US border.

“The government should be in the business of keeping families together, not tearing them apart,” Harris, a rising star in the Democratic Party, said in a statement. She added, “And the government should have a commitment to transparency and accountability. Under Secretary Nielsen’s tenure, the Department of Homeland Security has a track record of neither. As a result, she must resign.”

The Department of Homeland Security did not immediately return a request for comment on Harris’ resignation call.

Earlier on Monday, Nielsen defended the Trump administration’s immigration policy, saying, “We will not apologize for the job we do, or for the job law enforcement does for doing the job that the American people expect us to do.”

The Trump administration’s zero-tolerance policy at the border has resulted in thousands of family separations. The policy hasn’t deterred immigrants from trying to enter the country illegally, internal Department of Homeland Security documents obtained by CNN show Monday.

Harris has previously criticized the separation of undocumented children from their parents, calling it “human rights abuse” — and has frequently and famously butted heads with Nielsen and her predecessor John Kelly. But Monday’s statement marks the first time the Democratic senator has called for the DHS secretary’s resignation, her office confirmed to CNN.

The administration’s “zero-tolerance” policy has sparked a backlash from Democrats and condemnation from some Republicans as well, including former first lady Laura Bush, who called it “cruel” and “immoral” in a Washington Post op-ed.

In a series of tweets on Monday, Nielsen said, “Border security and immigration enforcement are complex topics” that can’t “be reduced to media soundbites.” The DHS secretary went on to call for Congress to take action to “secure our borders” and said, “I will not stand by while our immigration system continues to deteriorate.”