President-elect Trump has chosen Rep. Michael Waltz (R-Fla.) to serve as his national security adviser, tapping the former Green Beret to fill a top White House post, according to a person familiar with the decision.

Waltz, 50, has served in Congress since 2019. He has called for Europe to do more to support Ukraine and for the U.S. to be more stringent with its support, aligning with a key foreign policy goal of the president-elect. He has also been a staunch critic of the Biden administration’s 2021 withdrawal from Afghanistan.

The Wall Street Journal first reported on Trump’s choice of Waltz.

Waltz had been open about his willingness to serve in the administration and was considered a candidate to lead the Pentagon. The role of national security adviser does not require Senate confirmation.

Republicans are expected to have an extremely narrow House majority, and Waltz’s exit to join the administration will further complicate efforts to pass bills at the beginning of Trump’s term. Rep. Elise Stefanik (R-N.Y.) has also been tapped for an administration role, leaving two vacant GOP seats.

Trump went through multiple national security advisers during his first term. Michael Flynn was in the role for just a few weeks before resigning amid controversy over his conversations with Russia. Others to hold the post included H.R. McMaster, John Bolton and Robert O’Brien.

The former president has been quickly assembling a roster of senior staff since winning last week’s election. He has already announced his chief of staff, his “border czar,” and nominees for ambassador to the United Nations and Environmental Protection Agency administrator.