Final answer:
Out of the options listed, legal challenges do not typically serve as a tool for constituency-building since they involve judicial intervention rather than direct public engagement.
Step-by-step explanation:
The constituency-building strategy tools mentioned in the question are methods used by different groups to influence public policy and create a support base. Expert witness testimony, advocacy advertising, and public relations are all traditional means by which organizations try to shape public opinion and influence the policymaking process in their favor. Legal challenges, on the other hand, involve using the judicial system to influence public policy, but do not directly build a constituency in the way the other options do; instead, they can potentially create new legal precedents.