Amid a national emergency on the southern U.S. border, President Trump made two big changes at the Department of Homeland Security.

On Friday, the White House pulled the nomination of Ron Vitiello to be the permanent director of U.S. Immigration and Customs Enforcement after he served as the acting director of the agency since June 2018. A couple of days later, Trump fired DHS Secretary Kirstjen Nielsen, a move that took many by surprise but was widely welcomed by immigration hardliners.

On Monday, Nielsen told reporters she hopes to support Trump's agenda from the outside, saying, "I share the president’s goal of securing the border.”

The concept of “securing the border” has been in dispute. The southern U.S. border has been far from secure for years. It’s one of the reasons why Trump is currently president as he ran on a policy platform of building a wall.

But matters have gotten so bad under Nielsen, in particular, Trump declared a national emergency in February 2019. Who could possibly be the next DHS secretary?

Secretary of Energy Rick Perry, who was a three-term governor of Texas, is one of the top contenders for the position. However, he’s expressed skepticism that a physical border wall should be built, saying in 2011 during his presidential run, "If you build a 30-foot wall from El Paso to Brownsville, the 35-foot ladder business gets real good.” In 2016, Perry walked back that skepticism, embracing Trump’s proposal to build a wall.

It wouldn’t be shocking if Trump picks Perry. He was confirmed as energy secretary by the Senate, 62 to 37. So, there’s little concern from the White House that he can get confirmed this time around. But there are other potential nominees Trump could pick who might have a more difficult time.

Kris Kobach, the former Kansas secretary of state and longtime Trump ally, has been on the White House’s radar for some time, seeing as how he’s in lockstep with Trump on immigration, but his nomination would likely tank in the Senate. Then there’s Ken Cuccinelli, the former Virginia attorney general, who’s also been supportive of Trump but has few allies in the Republican-controlled Senate after demanding Senate Majority Leader Mitch McConnell step down in 2017.

Of course, Kevin McAleenan, who is serving as the acting secretary of Homeland Security, jumping over as commissioner of Customs and Border Protection, shouldn’t be ruled out as a permanent replacement. McAleenan carried out Trump’s “zero tolerance policy” for border crossings in the summer of 2018 that led to families being separated.

Like all things happening in the country, the buck stops with the president. Nielsen may not have met the expectations set out by immigration hardliners, but Trump is ultimately responsible for allowing the situation along the southern U.S. border to worsen.