After Acting Attorney General Sally Yates announced that she had ordered the U.S. Department of Justice not to enforce President Donald Trump’s recent executive order limiting immigration from terrorist safe havens, Trump immediately fired Yates and announced her temporary replacement. Yates took over the top Justice job following Loretta Lynch’s exit and pending Senate confirmation of Jeff Sessions, whose nomination is still pending in the Senate.

Josh Blackman, a law professor at the South Texas College of Law in Houston, explained on Twitter on Monday evening that Trump has clear authority to fire political appointees like Yates. Blackman further noted that if Yates truly felt the law was unconstitutional or legally unenforceable, she had an obligation to resign her position rather than simply refuse to do her job.

“Duty to take care that the laws are faithfully executed belongs to POTUS alone,” Blackman explained. “If appointee gets in the way, removal is the only option.”

“The notion that an official could simply refuse to follow President’s order, rather than resign, is itself contrary to [separation] of powers,” he continued. “The President’s oath to support & defend the Constitution matters. If his appointees (or worse, holdovers) disagree, they should resign.”

Yates was a holdover from the Obama administration. Trump announced that she would be replaced by U.S. Attorney Dana Boente pending Senate confirmation of Sessions.

You can read Blackman’s full explanation from Twitter below.

1/ The Federal Vacancies Act permits POTUS to replace Senate-confirmed officer with another Senate-confirmed Officer https://t.co/Qn62F9ZuAb — Josh Blackman (@JoshMBlackman) January 31, 2017

2/ Here is key section from 5 U.S.C. 3345(b), which permits the President to appoint another officer in an acting capacity for limited time pic.twitter.com/eigj2pOyZv — Josh Blackman (@JoshMBlackman) January 31, 2017

3/ How did I put this together so quickly? I took it from a draft blog post I wrote about repealing and replacing Richard Cordray. — Josh Blackman (@JoshMBlackman) January 31, 2017

4/ The key point is,as @jadler1969 noted, is that this decision trumps any extant Obama EO about chain of succession https://t.co/L25q6MM2q8 — Josh Blackman (@JoshMBlackman) January 31, 2017

5/ There is residual question about what powers an Acting AG can perform. But the same analysis that applies to Yates now applies to Boente — Josh Blackman (@JoshMBlackman) January 31, 2017

6/ Some people have asked about approving FISA warrants. I don't know the answer to that. — Josh Blackman (@JoshMBlackman) January 31, 2017

7/ Yates should have resigned if she did not want to defend the Executive Order–especially of OLC approved and Civil could defend it — Josh Blackman (@JoshMBlackman) January 31, 2017