From their press release (note that the question refers to future large-scale illegal immigration, not those currently here):

A recent Harris Poll asked a cross-section of U.S. adults to say how likely they think it is that various possible events would be "a major threat to the United States in the next five years." Two of these possible threats are seen as extremely or very likely by over half of the public:

* 55 percent of all adults think it is extremely or very likely that a large number of illegal immigrants coming into this country would be a threat;

* 52 percent of adults believe that a significant loss of jobs to foreign countries would be an extremely or very likely threat.

These are some of the results of a Harris Poll of 1,508 U.S. adults, surveyed online between January 5 and 12, 2007. However, because split samples were used, each individual item on the list was asked of between 400 and 500 adults.

The only other possible event seen as an extremely or very likely threat by more than 40 percent of all adults is a significant natural disaster destroying areas of a major city (43%)...