President Trump revealed in a tweet Sunday that U.S. Customs and Border Protection (CBP) Commissioner Kevin McAleenan will become the acting head of Homeland Security, the sprawling department of 240,000 people, following the resignation of Secretary Kirstjen Nielsen.

McAleenan is a longtime border officer, reflecting Trump’s priority for the department initially founded to combat terrorism after the Sept. 11 attacks. Fox News is told Trump wanted the “toughest cop” around on border security, and McAleenan fit the bill.

Administration sources tell Fox News that McAleenan needs to be able to handle the politics involved and change minds on Capitol Hill before he could be seen as potentially taking the job of DHS secretary permanently.

Last month, McAleenan, who as head of CBP was the nation’s top border security official, said the border crisis was at its "breaking point" during a visit to Texas. He reiterated that there are not enough agents to respond.

"That breaking point has arrived this week at our border," McAleenan said along the border. "CBP is facing an unprecedented humanitarian and border security crisis all along our Southwest border, and nowhere has that crisis manifested more acutely than here in El Paso."

Prior to government service, according to his bio, McAleenan practiced law in California. He received his Juris Doctor degree from the University of Chicago Law School and a Bachelor of Arts degree from Amherst College.

CLICK HERE TO GET THE FOX NEWS APP

McAleenan is well-respected by members of Congress and within the administration, and beyond.

“Significant leadership changes today for DHS and CBP. I’m confident Mr. McAleenan’s experience and knowledge will help us better address the current crisis on our southern border. #DHS #BorderCrisis #BorderPatrol #CBP” tweeted U.S. Border Patrol official Matthew Hudak.

Fox News' John Roberts and the Associated Press contributed to this report.