In his post “Beating the Iron Law of Public Choice: A Reply to Peter Boettke,” Lars Christensen gives this description of the supposed Iron Law of Public Choice: 

… the Iron Law of Public Choice – no matter how much would-be reformers try they will be up against a wall of resistance. Reforms are doomed to end in tears and reformers are doomed to end depressed and disappointed.

Lars gives this ancestry for the Iron Law of Public Choice: 

The students of Public Choice theory will learn from Bill Niskanen that bureaucrats has an informational advantage that they will use to maximizes budgets. They will learn that interest groups will lobby to increase government subsidies and special favours. Gordon Tulluck teaches us that groups will engage in wasteful rent-seeking. Mancur Olson will tell us that well-organized groups will highjack the political process. Voters will be rationally ignorant or even as Bryan Caplan claims rationally irrational.

 But at the end of the day, I agree with Lars when he says

ideas – especially good and sound ideas – can beat the Iron Law of Public Choice.