I got these data series from Fred, but I am too dumb to get Fred to graph the ratio so I put them into Excel, created the ratio, graphed it and exported to this blogpost. Lost the horizontal axis somewhere along the way but it runs from January 1939 to March 2016. The exact variables are listed below the graph.
People, Manufacturing share of Total non farm employment peaked in November of 1943 (the great war!!) and has by and large been falling ever since.
It's hard for me, given the historical trend, to put too much blame for the relative shrinking of manufacturing employment on the rise of China, or NAFTA, because it was happening in the 1960s and 1970s well before either of these things happened.