That's because the majority of migrants arriving now are families and children fleeing violence and poverty in Central America. Unlike individual migrants looking for work, many of these migrants are seeking asylum and must be allowed to pursue their claims in immigration court.
Overwhelmed by the influx, Customs and Border Protection has begun releasing more families with a notice to appear in immigration court instead of first handing them over to Immigration and Customs Enforcement for processing and detention.
Nielsen tried to shift resources within her department to help with processing the migrants, and last week appointed a Border Patrol veteran to serve as coordinator of the department's response to the "border crisis."
"I want to be clear with the American people: there is an unprecedented emergency at the southern border, and DHS is leading a true government-wide emergency response," Nielsen said in a statement announcing the move.
Nielsen also joined President Trump in calling on Congress to close what they call "loopholes" in immigration law to make it easier to detain and deport migrant families and kids.
On Defense With Trump — and For Him
Throughout her tenure, Trump blamed Nielsen for an inability to get Congress to fully fund a wall on the southern U.S. border with Mexico, even though she was not involved in the spending deal Trump reached with congressional leaders earlier in 2018. The border wall funding question led to a stalemate that caused the longest partial government shutdown in U.S. history — with the president following through on his earlier threats not to sign critical spending bills unless he gets what he deems sufficient funding for his border wall and with congressional Democrats rejecting any legislation that would provide such funding.
Democrats and immigration advocates also criticized Nielsen for policies she advocated that actively sought to separate children from their parents at the border, including a protest inside a Mexican restaurant in Washington where the secretary was eating dinner last June.
"The law says if you cross between the ports of entry, you are entering without inspection and that is a crime," Nielsen said, defending herself in an interview with NPR's John Burnett in May 2018.
"Our policy has not changed in that if you break the law, we will refer you for prosecution," she added. "What that means, however, is if you are single adult, if you are part of a family, if you are pregnant, if you have any other condition, you're an adult and you break the law, we will refer you. Operationally what that means is we will have to separate your family."
The administration backed away from that policy after a public outcry and in light of court rulings — just days after Nielsen had mounted a vigorous public defense of the controversial policy.
Nielsen had to defend Trump on other issues, too.
In January 2018, a little over a month after she was sworn in as DHS secretary, Nielsen testified to a Senate panel that she had not heard Trump describe some African nations as "shithole countries," although she conceded that he had used "tough language" in a meeting on immigration issues that included members of Congress who insisted Trump had, in fact, used the pejorative.
Months later, in response to a reporter's question, she said she "was not aware of" reports from U.S. intelligence agencies that concluded that the Russian government actively tried to swing the 2016 presidential election to Trump and away from Democrat Hillary Clinton.
Nielsen's characterization of the foreign interference threat to U.S. elections evolved: She later said that although it continued to happen, it was not taking place with the same intensity as it had in 2016.
DHS and other agencies released a statement ahead of Election Day 2018 that said the focus of foreign influence operations was on disinformation via social media platforms and less on cyberattacks aimed at stealing embarrassing information about political targets or disrupting the actual working of elections.