Didn't really look for it.  Like I said, my point isn't that the law is unconstitutional, but rather that arguing that voluntarily compliance trumps constitutionality is flawed.

Regardless, the case isn't really dispositive.  It lays a good foundation for the argument in support of the constitutionality, but it's focused on the actions of a political party in selecting candidates in a primary.  The lack of state action is critical to its decision.  There's a pretty fundamental difference between a political party saying "we won't put you on the ballot unless you promise to vote a certain way" and a state government saying "we're going to engage in criminal prosecution if you don't vote a certain way."