Technically I believe the United States is a Constitutional Republic.
Danger: anecdote ahead...I listened to an NPR interview recently where it was stated there was significant fear during the Irish/Italian immigration waves that the immigrants were not capable of appreciating US' liberty, and would effectively dilute it. I now think that was accurate, and came to pass.
One cannot argue that in today's United States we have liberty - cutting down a tree requires a permit, even when there are no safety considerations. Growing various plants is illegal. Operating a hair-cutting business without the proper permits is illegal. The list goes on...
Republic? Long gone... One can debate, but I would nominate Wilson at the latest. FDR is the common scapegoat, but Wilson certainly set the stage. There may be earlier contestants, but this is not my area of expertise.