Former colleagues said on Wednesday that Ms. Nielsen was well qualified.
âSheâs a total homeland security expert â absolutely has no learning curve,â said Michael Allen, who worked with Ms. Nielsen during the Bush administration. âSheâs an experienced manager, sheâs an implementer, she knows how to get under the hood and figure out what needs to be connected to what.â
Added Frances Townsend, her boss at the White House during the Bush administration: âShe is tough as nails, competent and has rightly earned the presidentâs respect.â
Mr. Kelly and Ms. Nielsen arrived at the White House after a tumultuous period that led to the departure of Reince Priebus, Mr. Trumpâs first chief of staff, and Sean Spicer, his original press secretary. Mr. Kelly, who spends much of his time managing the president and working to defuse brewing confrontations between Mr. Trump and senior advisers, has leaned on Ms. Nielsen to manage and impose discipline on the rest of the White House staff.
It is not clear who will play that role after she leaves, and her departure will leave a hole at the White House as Mr. Trump faces an array of challenges, including confronting the threat from North Korea, addressing ruptures with his cabinet and crucial Republican lawmakers and trying to push a large tax cut through Congress.
Ms. Nielsenâs experience at the Homeland Security department began after the Sept. 11 attacks, when she joined the newly formed Transportation Security Administration, housed within the department. She also served on Mr. Bushâs homeland security council, devising a national response plan for domestic incidents and helping to compile a report on lessons from the bungled federal response to Hurricane Katrina.
As a private-sector consultant, she helped government entities and private companies devise preparedness plans to guard against cyberattacks.
Tom Ridge, who served under Mr. Bush as the…