Economics is always a part of this equation. The belief is that many immigrants enter America and take jobs that the native- born Americans could have. Certainly, the rise of Nativism in the American period of industrialization was a part of this. The modern version of nativism is not as open about its economic motives, but rather uses the law and its enforcement as a part of this resentment. The fear of immigration is channelled through its illegal entry component and argues that the breaking of the rules and the risk to security as the reasons why there is resentment. Certainly, this is true. However, within this sphere of individuals is also a contingent that still feels intrinsic resentment about immigration in America, in general, and uses the defenses of natural security and legality as a way to shroud this resentment.
It's partly human nature, and today's fear of and backlash against immigrants is not unlike what's happened during past waves of newcomers like the Poles, Jews, Russians and Italians, not to mention the Irish.
The main reason is because people are unfamiliar with the new culture, and see it as a threat to their own. They see signs and hear conversations in a language they don't understand and don't want to and it makes them uncomfortable. Also, when economic times are bad, immigrants are an easy target for both politicians and natives, and since they are not part of the mainstream culture yet, immigrants have a hard time defending themselves or becoming more accepted by society.
There have been at least two times that this has happened and I am not clear as to which one you are talking about.
In the 1840s, the major problems that "native" Americans had with the immigrants included:
- Their religion. The Catholic religion was seen by many Americans as an anti-democratic faith. They were afraid the immigrants would be loyal to the pope and not to the US.
- Their lack of education.
- Their poverty.
There was a great deal of concern that these factors would prevent the immigrants from truly assimilating and becoming "real" Americans.