I'd go back and blame the downfall on 1913...
1913 brought us the Federal Reserve, the income tax (Via the 16th amendment) and moving election of US senators away from state legislatures to direct elections (Via the 17th amendment)
Repeal 1913 and we'd be a whole lot better off...

Since then each admin has chipped away at the basic freedoms and structure of the US as it was envisioned and created. Even to the structure and practice of laws in the US.
I would think that the actual date to blame it on may be a bit further back since the plans and execution of them would have to be well thought out. Its not as if this was thought of and planned in an instant because of its far reaching nature. Repealing 1913 would be a tremendous start but I would like to go back and see the backroom deals that led up to this.
Somewhere between 1776 and 1913 England and these planners managed to get their hooks back into America. It was 1776 that America as we know it today was formed but it was not the start of America as it was envisioned by those that started it. That started well back with the Mayflower Compact which was truly the founding of America as it was supposed to be.