Mercantilism is the ideology that nations must protect their wealth from infringement by other countries using techniques such as tariffs. The idea reached its zenith in the 1600s as European countries battled each other for economic superiority using tariffs and colonial trade restrictions. Underneath these techniques is the idea that our country has more of a right to wealth than other countries, and we should restrict our trade with those other countries to come out on the winning side. This same tendency has come up in the immigration reform debate: every immigrant that crosses the border is taking one of "our" jobs.
Today, economists have largely abandoned the idea of mercantilism, recognizing it makes no sense. Mercantilism depends on the idea of a zero sum pie, your gain must mean my loss. A mercantilist would say that I have a trade deficit with my local grocery store--they have all my money, and I'm left penniless. Of course this ignores that I'd die of starvation if I didn't have these valuable groceries. Trade that's free of coercion and fraud follows this same pattern of mutual benefit.
Today, economists use the word mercantilism as an insult (http://en.wikipedia.org/wiki/Mercantilism#CITEREFWilson1963). Politicians have not been as wise, claiming that many of us will be left unemployed if there was open immigration. This is the same limited pie idea that underlies mercantilism. In truth, there's not a finite supply of jobs.
The largest labor union, ALF CIO outlines its action plan for immigration, and highlights the need to limit immigration as a means to protect American jobs, calling for "an independent commission to assess and manage future flows, based on labor market shortages that are determined on the basis of actual need." They also call for "improvement, not expansion, of temporary worker programs, limited to temporary or seasonal, not permanent, jobs." So according to these Democrat backers anyone who's not a natural born American apparently doesn't have a right to compete for a permanent job here.
The conservative group ALIPAC says, "America is a land of generous and caring people, but our hospitality and values are being strained and abused by those who are willing to break the law and take our jobs and our tax dollars." I don't remember The Declaration of Independence saying anything about the inalienable right to a job that no other person can compete for.