BYU law professor Aaron Nielson is a renowned scholar and “a key member of the conservative legal movement” with expertise litigating against alleged regulatory overreach by federal authorities, according to lawyers and academics familiar with his work and even those who have battled him in court. The new Texas solicitor is an expert on the Administrative Procedure Act, a 1946 law that governs how federal agencies develop and implement regulations and a statute that Paxton has repeatedly used to challenge the authority of federal agencies. But in this article, The Texas Lawbook examines Nielson’s work on a 2020 U.S. Supreme Court case in which he was appointed by the justices to, ironically, brief and argue in favor of the constitutionality of the Federal Housing Finance Agency’s structure — a position that legal experts agree his boss, Texas AG Ken Paxton and other anti-federal agency advocates opposed. Nielson’s arguments won the day.