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President Donald Trump on Thursday introduced his choice to lead the Department of Homeland Security, a former staffer at the sprawling federal agency who he says will need "no on-the-job training" to take on the lead role.

Trump called on Congress to "put politics aside" and confirm deputy White House chief of staff Kirstjen Nielsen by a "strong, bipartisan vote."

But even before Trump formally announced Nielsen's appointment during an East Room ceremony that was attended by much of the Cabinet and senior members of the White House staff, the top Democrat on a key committee signaled that he had questions about Nielsen's background.

Her nomination must be confirmed by the Senate.

"There will be no on the job training for Kirsten. She is ready on Day One," Trump said.

Nielsen told the president she was "humbled by the trust you are placing in me." If confirmed by the Senate, she will become the sixth secretary of a department that was created in the wake of the 9/11 terrorist attacks.

Nielsen had been chief of staff to John Kelly when he was Trump's first homeland security secretary. Kelly brought Nielsen to the White House when Trump named him chief of staff in late July, and Trump quickly tapped her to be Kelly's deputy.

An expert in homeland and national security policy, Nielsen previously served as a special assistant to President George W. Bush and worked for the Transportation Security Administration.

The White House said Wednesday in a written announcement that Nielsen "has extensive professional experience in the areas of homeland security policy and strategy, cybersecurity, critical infrastructure, and emergency management." She's also the first nominee to have previously worked for the department.

Nielsen, however, has drawn some resentment in her current White House position, as she and Kelly have tried to instill more discipline at the White House. That has included limiting access to the president to some who had formerly enjoyed near-unimpeded access.

Rep. Bennie G. Thompson of Mississippi, the senior Democrat on the House Committee on Homeland Security, said Wednesday in a statement he was pleased the president had made a decision on filling the post "after letting the critical national security position remain vacant as the nation faced multiple major hurricanes and a domestic terrorism attack."

Thompson said he had questions about Nielsen's background, including her past work for the Bush administration.

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