Federal Rule of Civil Procedure 5.1
Rule 5.1. Constitutional Challenge to a Statute—Notice, Certification, and Intervention
(a) Notice by a Party. A party that files a pleading, written motion, or other paper drawing into question the constitutionality of a federal or state statute must promptly:
(1) file a notice of constitutional question stating the question and identifying the paper that raises it, if:
(A) a federal statute is questioned and the parties do not include the United States, one of its agencies, or one of its officers or employees in an official capacity; or
(B) a state statute is questioned and the parties do not include the state, one of its agencies, or one of its officers or employees in an official capacity; and
(2) serve the notice and paper on the Attorney General of the United States if a federal statute is questioned—or on the state attorney general if a state statute is questioned—either by certified or registered mail or by sending it to an electronic address designated by the attorney general for this purpose.
(b) Certification by the Court. The court must, under 28 U.S.C. §2403, certify to the appropriate attorney general that a statute has been questioned.
(c) Intervention; Final Decision on the Merits. Unless the court sets a later time, the attorney general may intervene within 60 days after the notice is filed or after the court certifies the challenge, whichever is earlier. Before the time to intervene expires, the court may reject the constitutional challenge, but may not enter a final judgment holding the statute unconstitutional.
(d) No Forfeiture. A party’s failure to file and serve the notice, or the court’s failure to certify, does not forfeit a constitutional claim or defense that is otherwise timely asserted.