The case is somewhat interesting in that the plaintiffs have raised constitutional objections to the ACA's creation of the Independent Payment Advisory Board (IPAB), the body established to study and issue reports on Medicare provider payments. Specifically, the plaintiffs claim that the ACA "fast track" provisions concerning how Congress is to vote on IPAB proposals unconstitutionally "entrenches" the IPAB against repeal by future Congresses, and that the ACA's grant of discretion to IPAB runs afoul of the non-delegation doctrine.
In its motion, the United States argues that the plaintiffs lack standing, as well as that their complaint fails to state any viable claims on the merits.
You can access the United States's motion to dismiss here, and you can access the plaintiffs' amended complaint here.
UPDATE: The specific "entrenchment" claim that the plaintiffs have raised is this (Count VI of their amended complaint):
The Act's entrenchment of IPAB's proposals, recommendations and enabling statutes from future modification, amendment or repeal by future congresses, causes irreparable injury by violating the First Amendment voting rights of Plaintiffs U.S. Representatives Flake and Franks and other federal legislators.The Supreme Court's decision Monday in Nevada Commission on Ethics v. Carrigan, which held that a legislator's vote on legislation is not "speech" for purposes of the First Amendment, likely renders this claim untenable.