(CNN) President Donald Trump's choice to fill the role of Kirstjen Nielsen as secretary of Homeland Security is a career official who served in the Obama administration and whom a senior DHS official says is "not an ideologue or fire breather" on immigration.

Kevin McAleenan, who is expected to take over DHS in an acting capacity, was serving as the commissioner of US Customs and Border Protection before Sunday's announcement. He was confirmed as commissioner by the Senate in March 2018 by a vote of 77-19.

He is expected to serve as the acting secretary "in the short term," according to a White House official. Nielsen should be staying for a week of transition, another White House official said.

Nielsen tweeted Sunday night that she "agreed to stay on as Secretary through Wednesday, April 10th to assist with an orderly transition and ensure that key DHS missions are not impacted."

Removing Nielsen and installing McAleenan as acting secretary presents issues for the line of succession. Putting McAleenan in charge skips over acting Deputy Secretary Claire Grady, who also serves in the Senate confirmed role as the Undersecretary of Management, which raises questions about her future. A senior administration official told CNN that legally Grady is next in line and will have to go in order for McAleenan to assume the role of acting secretary. Another official, asked about this question, told CNN there are issues that need to be addressed before McAleenan can take over.

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