Daniel Webster: "Protection...of our own labor against the cheaper, ill-paid, half-fed, and pauper labor of Europe, is, in my opinion, a duty which the country owes to its own citizens."
You know, today we would say "cheaper, ill-paid, half-fed, and pauper labor of China".
Without protectionism, American jobs which have, say, $2 per hour wages, cannot compete with Chinese labors with $2 per day wages. If General Motors were to decide where they should build an auto plant, without protectionism they will have more reasons to move their factories to China rather than continuing their local factory in America. (This is also the reason why most of our goods are MADE IN CHINA) Protectionism protects the local jobs by requiring tariffs on foreign-made goods to prevent foreign labors and foreign businesses from overtaking, and eventually destroying, local jobs. It also encourages the growth of local industry (along with free-market system, of course. Otherwise, it is no different from state capitalism (a.k.a socialism)).
Free Trade may be good for China (and other less developed countries), but it is not good for America which has higher standard of living.