Two great quotations:

But the Policy Requirement goes beyond preventing recipients from using private funds in a way that would undermine the federal program. It requires them to pledge allegiance to the Government’s policy of eradicating prostitution. As to that, we cannot improve upon what Justice Jackson wrote for the Court 70 years ago: “If there is any fixed star in our constitutional constellation, it is that no official, high or petty, can prescribe what shall be orthodox in politics, nationalism, religion, or other matters of opinion or force citizens to confess by word or act their faith therein.” Barnette, 319 U. S., at 642.

I love how the Chief used the pledge of allegiance in this context. And this gem from Cardozo:

As noted, the distinction drawn in these cases—between conditions that define the federal program and those that reach outside it—is not always self-evident. As Justice Cardozo put it in a related context, “Definition more precise must abide the wisdom of the future.” Steward Machine Co. v. Davis, 301 U. S. 548, 591 (1937). Here, however, we are confident that the Policy Requirement falls on the unconstitutional side of the line.

Update: Scalia is not impressed with citations to Barnett:

The Court’s opinion contains stirring quotations from cases like West Virginia Bd. of Ed. v. Barnette, 319 U. S. 624 (1943), and Turner Broadcasting System, Inc. v. FCC, 512 U. S. 622 (1994). They serve only to distract attentionfrom the elephant in the room: that the Government is not forcing anyone to say anything. What Congress has done here—requiring an ideological commitment relevant to the Government task at hand—is approved by the Constitution itself. Americans need not support the Constitution; they may be Communists or anarchists. But “[t]he Senators and Representatives . . . , and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support [the] Constitution.” U. S. Const., Art. VI, cl. 3. The Framers saw the wisdom of imposing affirmative ideological commitments prerequisite to assisting in the government’s work. And so should we.

Update 2: This is only the tenth time the Oath clause has been cited by the Supreme Court. The last time was in Printz.