Kirstjen Nielsen, the homeland security secretary, resigned on Sunday after meeting with President Trump, ending a tumultuous tenure in charge of the border security agency that had made her the target of the president’s criticism.
“I have determined that it is the right time for me to step aside,” Ms. Nielsen said in a resignation letter. “I hope that the next secretary will have the support of Congress and the courts in fixing the laws which have impeded our ability to fully secure America’s borders and which have contributed to discord in our nation’s discourse.”
Ms. Nielsen had requested the meeting to plan “a way forward” at the border, in part thinking she could have a reasoned conversation with Mr. Trump about the role, according to three people familiar with the meeting. She came prepared with a list of things that needed to change to improve the relationship with Mr. Trump.
Mr. Trump in recent weeks had asked Ms. Nielsen to close the ports of entry along the border and to stop accepting asylum seekers, which Ms. Nielsen found ineffective and inappropriate. While the 30-minute meeting was cordial, Mr. Trump was determined to ask for her resignation. After the meeting, she submitted it.
The move comes just two days after Mr. Trump, who has repeatedly expressed anger at a rise in migrants at the southwestern border, withdrew his nominee to run Immigration and Customs Enforcement because he wanted the agency to go in a “tougher” direction.
Mr. Trump has ratcheted up his anti-immigration message in recent months as he seeks to galvanize supporters before the 2020 election, shutting down the government and then declaring a national emergency to secure funding to build a border wall, cutting aid to Central American countries and repeatedly denouncing what he believes is a crisis of migrants trying to get into the country.
He took aim again Sunday night after announcing Ms. Nielsen’s departure, tweeting, “Our Country is FULL!”
Ms. Nielsen’s resignation was effective immediately, according to the letter. The abruptness was unusual because the Department of Homeland Security currently does not have a deputy secretary, who would normally take the reins. The president said in a tweet that Kevin McAleenan, the commissioner of Customs and Border Protection, would take over as the acting replacement for Ms. Nielsen, who became the sixth secretary to lead the agency in late 2017.
Among the possible replacements for Ms. Nielsen in the long term is Ken Cuccinelli, the former Virginia attorney general who is a favorite among conservative activists and who fits the profile that Mr. Trump wants the next homeland secretary to have, people familiar with the discussions said.
Ms. Nielsen had been pressured by Mr. Trump to be more aggressive in stemming the influx of migrant crossings at the border, people familiar with their discussions in recent months said.
Her entire time in the job was spent batting back suspicion from the president, even as he told people he liked how she performed on television and enjoyed dealing with her personally. He initially was skeptical because of Ms. Nielsen’s previous service in the George W. Bush administration, and then because she was close to John F. Kelly, the president’s former chief of staff.
The president called Ms. Nielsen at home early in the mornings to demand that she take action to stop migrants from entering the country, including doing things that were clearly illegal, like blocking all migrants from seeking asylum. She repeatedly noted the limitations imposed on her department by federal laws, court settlements and international obligations.
Those responses only infuriated Mr. Trump further. The president’s fury erupted in the spring of 2018 as Ms. Nielsen hesitated for weeks about whether to sign a memo ordering the routine separation of migrant children from their families so that the parents could be detained.
In a cabinet meeting surrounded by her peers, Mr. Trump lambasted her repeatedly, leading her to draft a resignation letter and tell colleagues that there was no reason for her to lead the department any longer. By the end of the week, she had reconsidered and remained in her position, becoming an increasingly fierce supporter of his policies, including the family separations.
Mr. Trump and Stephen Miller, his top immigration adviser, have privately but regularly complained about Ms. Nielsen. Lou Dobbs, a Fox News host who is one of the president’s favorite sounding boards, has also encouraged Mr. Trump’s negative views of her handling of the migrant crisis.
Mr. Trump and Stephen Miller, his top immigration adviser, have privately but regularly complained about Ms. Nielsen. Lou Dobbs, a Fox News host who is one of the president’s favorite sounding boards, has also encouraged Mr. Trump’s negative views of her handling of the migrant crisis.
On Friday, Mr. Trump traveled with Ms. Nielsen and Mr. McAleenan to Calexico, Calif., to describe what they called a crisis.
While the number of border crossings is not as high as in the early 2000s, the demographic of migrants has shifted largely from individual Mexicans looking for jobs — who could easily be deported — to Central American families, overwhelming detention facilities and prompting mass releases of migrants into cities along the border.
Ms. Nielsen estimated last month that border officials had stopped as many as 100,000 migrants in March.