I'm increasingly of the belief that free trade policies were popular in the United States because: 1. The expansion of communism was a serious threat 2. It was widely believed that the US was the greatest trading nation in the world. You want free trade if you're the best at it.
AFAICT, the only manufactured good that the United States sells to China that the US did not have to give up its IP for is planes, and now that Boeing is a mess, that is withering away too.