Held: Sections 16645.2 and 16645.7 are pre-empted by the NLRA. Pp. 4–16.

(a) The NLRA contains no express pre-emption provision, but this Court has held pre-emption necessary to implement federal labor policy where, inter alia, Congress intended particular conduct to “be unregulated because left ‘to be controlled by the free play of economic forces.’ ” Machinists v. Wisconsin Employment Relations Comm’n, 427 U. S. 132, 140. Pp. 4–5.