The promise appeared aimed to rebut suggestions that the Department of Homeland Security would initiate mass roundups of dreamers if Congress and the White House are unable to strike a deal to preserve the Deferred Action for Childhood Arrivals program President Trump terminated last fall.
Trump called the program, established by former president Barack Obama, "unconstitutional," and he set a March 5 deadline for lawmakers to act before the bulk of 690,000 work permits distributed to dreamers begin to expire. The congressional talks have stalled, with Trump rejecting a bipartisan Senate proposal last week.
But Nielsen's pronouncements have conflicted with other signals from the administration. In his first week, Trump signed an executive order that significantly broadened Obama-era enforcement priorities and stipulated that no groups of immigrants have a blanket exemption from deportation.
Immigration experts said that without specific policy safeguards, the dreamers are bound to become caught up in removal proceedings.
"There's a real disconnect between the political statements and what the written policies state," said John Sandweg, who served as acting general counsel at DHS during the Obama administration. "A few times, they've talked about prioritizing criminals. But the policy memos sent out to the field have been very clear that there is no distinction among anybody in the country unlawfully."
Nielsen, in an interview with "CBS This Morning" on Tuesday, emphasized that dreamers who do not commit crimes are "not going to be a priority of the Immigration and Customs Enforcement. I've said that before. That's not the policy of DHS."
But Nielsen added that DHS will "enforce the law."
To Doris Meissner, who served as commissioner of the Immigration and Naturalization Service under President Bill Clinton, the Trump administration's conflicting positions have obscured the significant shift in DHS's enforcement posture since the president took office.
"A large level of effort continues to go toward people who have criminal backgrounds, but the big difference is they have not precluded the idea of removing people who don't fall into those categories of criminals," Meissner said. "So that creates an entirely different atmosphere."
The question over how the United States should conduct enforcement operations against an estimated 11 million undocumented immigrants has become a crucial battleground as Congress has failed repeatedly over the past decade to enact a comprehensive legislative retooling of immigration laws.
After a bipartisan bill failed in 2007, the George W. Bush administration ramped up enforcement, conducting workplace raids that resulted in the apprehension of immigrants who had not committed crimes.
As deportation levels spiked to more than 400,000 a year in Obama's first term, his administration began to implement increasingly narrow guidelines aimed at prioritizing violent criminals and newly arrived immigrants, while allowing others to live largely without fear of deportation.
The legal rationale for the move was "prosecutorial discretion" — the concept that law enforcement agencies with limited resources must set priorities.
But Republicans revolted. In 2013, the GOP-controlled House approved an amendment, offered by Rep. Steve King (R-Iowa), to a government spending bill that would have forced DHS to scrap its discretion policies.
It is in this environment that DACA was born. Obama announced the program in 2012, arguing that immigrants who were low priorities for removal should be afforded the right to work and go to school.
A threat last year from Texas and several other Republican-led states to sue the Trump administration over DACA led Trump to rescind the program.
Nielsen's assurances that dreamers are a lower enforcement priority than other groups of undocumented immigrants conflict with the Trump administration's own legal analysis, said Simon Rosenberg, founder of the liberal think tank NDN.
"This is thin gruel — a vague assurance from the DHS secretary that 'we are not going after you,' " he said. "But there's also no ability for her to stop their deportation if they get into proceedings."
Mark Krikorian, executive director of the Center for Immigration Studies, which advocates for lower immigration levels, agreed that dreamers will be deported if they are apprehended.
But he added that "there is zero chance of a dragnet on people who lost their work permits. That's complete fiction."
Tyler Q. Houlton, a DHS spokesman, said that there is "no conflict" in DHS's enforcement priorities. The department, he said, "will continue to focus on criminal aliens, those with final orders of removal, and those who otherwise pose a threat to public safety and/or national security. However, DHS will not exempt entire categories or classes of people from potential immigration enforcement actions consistent with the law. A DACA recipient who loses their status, like all persons here illegally, is eligible for removal."
Democrats repeatedly pressed Nielsen to clarify her position during a lengthy Senate Judiciary Committee hearing on Tuesday. Citing a spike in the immigration arrests of non-criminals, Sen. Patrick J. Leahy (D-Vt.) raised the case of a Michigan father who arrived illegally from Mexico at age 10 and was deported this week after 30 years — based on an outstanding court order of removal from 2009.
"That's how we're using our limited-enforcement resources?" Leahy asked Nielsen. "Is it to strike fear in the hearts of everybody — whether they've done something wrong or not?"
Nielsen defended her agency, saying 92 percent of arrests made by ICE complied with DHS's enforcement priorities.
"Our statistics show that that is, in fact, what we're doing," she said, before adding: "I understand that there will always be exceptions."