How do organized interests respond to their opponents’ advocacy activities in a policy venue? Utilizing data on amicus curiae filings at the U.S. Supreme Court, the author estimates vector error correction and vector autoregression models that allow him test whether interests respond, in a dynamic sense, to the efforts of the “other side.” The author capitalizes on the temporal sequencing of variation in advocacy activity to gain leverage on the causal connection between the behaviors of opposing sets of interests and provides a richer portrait of the dynamics of interest representation in a policy venue. The results reveal that organized interests respond positively to the advocacy activities of their opponents by exhibiting both short-term counteraction and long-term countermobilization, implying that over the long run, interest representation at the Court is responsive and perhaps balanced.