But Nielsen had one crucial advantage - the absolute trust and support of Kelly, who she grew close to after volunteering to help "sherpa" him through the confirmation process earlier this year.
The White House said Nielsen has "extensive professional experience in the areas of homeland security policy and strategy, cybersecurity, critical infrastructure and emergency management".
After Kelly's move, the Cabinet-level DHS secretary position has gone unfilled, with Deputy Secretary Elaine Duke serving in the interim.
She is now the White House principal deputy chief of staff.
"She's a total homeland security expert - absolutely has no learning curve", said Michael Allen, who worked with Ms. Nielsen during the Bush administration.
If Nielsen's appointment is confirmed by the Senate, she would become the first former employee to run the department established after the terror attack on September 11, 2001.
At the White House, however, as Kelly's enforcer, Nielsen quickly emerged as a controversial presence. "It's a little concerning that she seems to have little background in immigration security and policy, but those individual agencies are in good hands already, and there is a strong core of career managers", said Jessica Vaughan, director of policy studies at the Center for Immigration Studies, which favours more limits on immigration. However, this was not her first time working in the White House, as she previously served in President George W. Bush's administration as the special assistant for prevention, preparedness and response on the Homeland Security Council.
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