Because he has the same problem with foreiengers and he is fear mongering the American public that the immigrants are taking away their jobs, corrupting their neighbourhoods and infesting like termites. He wants to deport those who he can, and stop it for others and some sections see him as messiah but they do not know that America is built on the foundation of immigrants from Italy, Ireland, India, Mexico etc. There wont be anyone doing those hard jobs that runs the country if it not for those mexicans or for those Indian doctors and scientiss and taxi drivers.
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