From the summary of Congressional Participation in Litigation: Article III and Legislative Standing (R45636, Mar. 26, 2019):
Houses, committees, and Members of Congress periodically seek to initiate or participate in litigation to, among other purposes, advance their legislative objectives, argue that the Executive is violating their legislative prerogatives, or defend core institutional interests. However, the constitutionally based doctrine of “standing” — which requires a litigant seeking federal judicial relief to demonstrate (1) a concrete and particularized and actual or imminent injury in fact, (2) that is traceable to the allegedly unlawful actions of the opposing party and (3) that is redressable by a favorable judicial decision — may prevent legislators from pursuing litigation in federal court. The U.S. Supreme Court and the lower federal courts have issued several important opinions analyzing whether — and under what circumstances — a legislative entity has standing to seek relief.