Nielsen defended the policy in a series of tweets on Sunday night; earlier in the weekend, her agency said it had separated almost 2,000 children from adults over the course of six weeks at the us southern border.
Nielsen said her department would continue enforcing the law, unlike past administrations.
"Illegal actions have and must have consequences", she said.
Harris cited, in particular, Nielsen's claim on Twitter Sunday evening denying that the Homeland Security Department had a policy of separating families at the border.
While Nielsen is technically correct - there's no statute explicitly calling for families to be forcibly separated - it has been the practical and direct outcome of this zero-tolerance policy.
"We do not separate breastfeeding children from their parents", the DHS official said. That is correct. But under USA protocol, if parents are jailed, their children are separated from them because the children aren't charged with a crime.
Trump administration officials have argued repeatedly in recent weeks that Democrats are responsible for the separation policy in an apparent reference to Congress' inability to pass an immigration bill that would provide funding for Trump's proposed border wall.
"I say it's very strongly the Democrats' fault, their obstruction, they're really obstructionist and they're really obstructing", he said. But in the meantime, he said, "The United States will not be a migrant camp and it will not be a refugee holding facility". "We do not want adults to bring children into this country unlawfully either, placing those children at risk".
One deputy killed, another in critical condition in Kansas City, Kansas
KSHB-TV reports the two deputies were transporting prisoners around 11:30 a.m. when they were "overcome" and were shot. Authorities said it was too early in the investigation to publicly detail how the shootings could have happened.
"Where is the outcry for the killings and crime being caused by gangs and thugs, including MS-13, coming into our country illegally?" he added.
The current policy stems from the Trump administration's "zero tolerance" toward people crossing the border illegally. "It is now time that Congress act to fix our broken immigration system", Nielsen said.
"We no longer exempt entire classes of people who break the law", Nielsen said of law changes. "That can not be the answer".
"Why is the government only releasing images of the boys being held?" a reporter, likely Kristen Welker of NBC News, asked the DHS Secretary at a White House press conference Monday afternoon.
"These children are not entering at points of entry, but risky places", Sessions said.
What about immigrants who come through legal ports of entry?
USA officials said on Friday that almost 2,000 children were separated from adults at the border between mid-April and the end of May.
"I find that offensive", Nielsen shot back.
"Don't believe the press. If you smuggle aliens. we will prosecute you". Multiple reports have also contradicted Nielsen's claim that asylum-seeking families are not being separated. There's videos, there's TVs, I visited the detention centers myself.