He got us into the Vietnam Quagmire which caused us to go deep in debt, triggering Balance of payment issues. Then Nixon came and and helped push the ball rolling.
There have been three quantum leaps in the reach of the federal government, under Lincoln, FDR and LBJ. Obama would like to add his name to that list. Liberal historians rank those three as among our greatest presidents for that reason. Others may have differing opinions. Another leap in federal power took place with the passage of the Sixteenth Amendment, allowing full and unrestricted or apportioned income taxes. Priot to that, the congress had been limited it its taxation of income derived from property. In the ratification debates, proponets assured skeptics that federal taxation would never rise above low single digits.
The actual downfall? Hmmm. But predictions of the downfall started back in the 18th century I'd wager.
The downfall began with Woodrow Wilson. He gave us the income tax, the Federal Reserve, and the principle that it's America's duty to fight other people's wars (World War I). Had we stayed out of World War I there would have been a negotiated peace, no punitive Treaty of Versailles, no rise of the Nazis, and no World War II.
I know that Versailles led to Hitler, but how did we lead to Versailles? Got a nice, in a nutshell link?
We made it possible to force an unconditional surrender on the Germans. It was pretty much a stalemate before we jumped in.