President Donald Trump said Thursday on social media he was firing Homeland Security Secretary Kristi Noem and would name Oklahoma Sen. Markwayne Mullin to replace her.

Trump thanked Noem for her service, saying in his Truth Social post she “has had numerous and spectacular results (especially on the Border!)” and that she “will be moving to be Special Envoy for The Shield of the Americas, our new Security Initiative in the Western Hemisphere.”

He said Mullin would take over the position March 31.

Trump loves watching Mullin on TV, aides said, which played a role in the president’s decision to tap him for the position. Trump has called Mullin following combative interviews to praise him and White House staffers have often dispatched the senator to do cable news hits around big moments for the administration.

Noem learned of her firing as she was arriving at an event in Nashville, Tennessee, three sources familiar with the matter told CNN. Two of the sources said Trump called Noem directly to inform her of his decision.

She thanked the president in a social media post shortly after the announcement.