In my mind, Ngai's thesis is that the current climate that surrounds the debate of immigration into the United States is one of fear.  Ngai's argument revolves around the idea that immigrants are not passive objects in the discourse.  Rather, they are a part of it and their voices have to be acknowledged:

In an important sense, “are immigrants good or bad for us?” is the wrong question. It takes as its premise that immigrants are not part of “us.” The idea falsely posits that non-citizens are not part of American society and leaves them out of the discussion. The mass demonstrations of Mexicans and other immigrants in Los Angeles, Chicago, and other cities in 2006 were significant because they showed that immigrants are no longer content to be the object of discussion but have emerged as subjects with voice and agency.

Ngai's arguments are enhanced by the idea that prior to the attacks of September 11, then- President Bush was advocating a path to citizenship for all illegal immigrants in the United States.  This was supposed to be a definitive statement on immigration that was put aside because of the attacks and the fear that resulted in America.  In the end, Ngai's thesis in the article is that the issue has become politicized to a level that has obscured the fact that immigration is a part of American identity, part of its history, and reflective of the promises of the nation's founding.