Nielsen previously worked as chief of staff to John Kelly - Trump's current White House chief of staff -when Kelly served as Trump's first DHS secretary. Mr. Kelly had drafted Ms. Nielsen to be his chief of staff at the Homeland Security department, and when the president plucked him for the White House, he brought her as his No. 2.
"Kirstjen can hit the ground running and there won't be a learning curve", Ridge said. She served as a special assistant to former President George W. Bush.
"I share the president's profound commitment to the security of our country and the safety of the American people", Nielsen said during the ceremony.
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Nielsen added that at DHS she plans to continue to provide support to communities struck by the recent wave of natural disasters, promising to "remain fully engaged in the long recovery effort ahead of us". She also worked for Kelly when he served as secretary of DHS.
At a formal White House event to announce the nomination, Trump spoke highly of Nielsen and her "sterling reputation as a dedicated leader whose number one priority was the security of our citizens, not politics or ideology". On Wednesday, some of them described Ms. Nielsen's promotion as a solution to a toxic personnel situation, while others fretted privately that her departure would create a void at the White House that would be hard to fill.
Mr. Kelly pushed hard for her selection, making a personal appeal to Mr. Trump during a monthslong search process.
"However, I am very concerned about her past work in the Bush Administration during its botched response to Hurricane Katrina and am fearful that DHS - as well as this President - has not learned all the lessons from that tragedy given what we are seeing unravel in Puerto Rico and the U.S. Virgin Islands".