Does law exhibit a significant constraint on Supreme Court justices' decisions? Although proponents of the attitudinal model argue that ideology predominantly influences justices' choices, “hybrid models” posit that law and ideology exhibit discrete and concurrent effects on justices' choices. I offer a new conceptualization of legal constraint examining how legal rules permit varying degrees of ideological discretion, which establishes how strongly ideological preferences will influence justices' votes. In examining the levels-of-scrutiny legal doctrine, I posit theoretical models highlighting the differential constraining capacities of the strict scrutiny, intermediate scrutiny, and rational basis rules. I use a multilevel modeling framework to test the hypotheses within the context of the Grayned doctrine in free expression law. The results show that strict scrutiny, which Grayned applied to content-based regulations of expression, significantly constrains ideological voting, whereas intermediate scrutiny (applied to content-neutral regulations) and the low scrutiny categories each promote high levels of ideological voting.