President Donald Trump said Friday that Deputy White House Chief of Staff Stephen Miller will be promoted to Chief of Staff, replacing Susie Wiles. Announcing the move on Truth Social, Trump wrote that Miller would start in the position on Monday, and that Wiles—who the president praised as having done “an outstanding job”—would move to another role in the “Trump Political Operation.”

Miller has played a major role in driving the White House’s domestic policy agenda, particularly when it comes to immigration. He has worked for the president since the 2016 campaign, and is widely viewed as a hard-liner within the administration. The shake-up comes just three days after Republicans suffered devastating losses in the midterm elections.