What is the test for a plaintiff to sue their government employer for violating the First Amendment?

To state a claim against their government employer for violating the First Amendment, Plaintiffs must show (1) that they engaged in protected speech, (2) that the employer took an adverse employment action against them, and (3) that their speech was a substantial or motivating factor for the adverse action. Coszalter v. City of Salem, 320 F.3d 968, 973 (9th Cir.2003). The pivotal element in this case is the first one: whether Plaintiffs' speech

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was constitutionally protected. This inquiry presents a question of law, which we review de novo. Berry v. Dep't of Soc. Servs., 447 F.3d 642, 648 (9th Cir.2006).