The problem from the constitutional point of view is that it requires such extensive reporting of payor/payee reasons for the transaction with respect to political activities, lobbying activities, and organizing activities, as well as collective bargaining activities, that this could hamper the ability of a trade organization to conduct its business. That in itself would potentially violate the freedom of association to belong to an effective trade union, as well as freedom of expression regarding political speech.
On October 25th, 2012. See this statement in context.