Sidley Austin is officially diving into the House’s legal battles.

Three attorneys at the law firm—Virginia Seitz, Carter Phillips and Joseph Guerra—filed notices of appearance as attorneys for the House in the Sierra Club and American Civil Liberties Union’s lawsuit challenging the Trump administration’s diversion of military funds for border wall construction. A short time later, they filed an amicus brief in the case on behalf of the House, marking Sidley’s first substantive filing on the merits of the case in the congressional legal battle.