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In a statement issued on Monday, Senator Harris said Nielsen needs to resign "over the department's policy that has resulted in thousands of children being separated from their families, a lack of transparency and for making misleading statements."
Harris tweeted, "Under her watch, our government has committed human rights abuses by breaking up families along the southern border. And she has failed to be accountable to and transparent with the American people."
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Harris' demand comes a day after Nielsen tweeted, "We do not have a policy of separating families at the border. Period."
On Monday, Nielsen defended her department's immigration policies in a speech before the National Sheriffs' Association.
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"We do not have the luxury of pretending that all individuals coming to this country as a family unit are in fact a family," Nielsen said. "We have to do our jobs. We will not apologize for doing our job."
Senator Harris called Nielson's comments "disqualifying."
"We must speak the truth. There is no law that says the Administration has to rip children from their families, " Harris said. "This Administration can and must reverse course now and it can and must find new leadership for the Department of Homeland Security."
I’m calling on Kirstjen Nielsen to resign as Secretary of Homeland Security. Under her watch, our government has committed human rights abuses by breaking up families along the southern border. And she has failed to be accountable to and transparent with the American people.— Kamala Harris (@KamalaHarris) June 18, 2018
We do not have a policy of separating families at the border. Period.— Sec. Kirstjen Nielsen (@SecNielsen) June 17, 2018