One fundamental difference between corporations and unions on one side and other kinds of groups created to influence public opinion is that corporations and unions have one single over-riding interest--profit and their own very limited economic well-being. While individuals are moral actors with a mixture of many motivations, corporations want only to increase revenues and limit costs. For a corporation, investments in political speech are just another kind of investment. Something similar can be said about unions.
Allowing public opinion to be bought and sold like any other commodity is inherently corrupting to our politics. The ideology on which modern democracy is based has at its roots the idea that individuals can look past their own individual self-interest when exercising their political rights--like voting and speaking on public issues. Corporations not only can't do this--they shouldn't. Neither can unions.
I suspect the net effect of this will be a growth in cynicism about all these institutions and a growing distrust of politics.