Gridlock, my freinds....Gridlock. Gridlock is the secret weapon of domestic freedom.
This was designed into the government by the Founding Fathers in various ways. Our government was designed to act slowly, to prevent it from acting on emotion. Unfortunately, they do this all the time now with the president just whipping out an executive order and bypassing the legislative branch or with the legislature passing bills under the premise of "we must act today or the economy will collapse tomorrow", without proper debate or consideration.
I don't think there will be one specific act that will cause the government to turn against us (I think they already are, for that matter). I think we will just wake up one day and it will occur to us that we no longer live in a republic.