When will a federal court review a state court's application of an individual statutory aggravating or mitigating circumstance in a particular case?

When a federal court is asked to review a state court's application of an individual statutory aggravating or mitigating circumstance in a particular case, it must first determine whether the statutory language defining the circumstance is itself too vague to provide any guidance to the sentencer. If so, then the federal court must attempt to determine whether the state courts have further defined the vague terms and, if they have done so, whether those definitions are constitutionally sufficient, i.e., whether they provide some guidance to the sentencer.

Walton v. Arizona, 497 U.S. 639, 654, 110 S.Ct. 3047, 3057-58, 111 L.Ed.2d 511 (1990) (emphasis original).