Hardliners want to arrest and deport all illegals and build a fence to keep them out, while sympathizers seek a path to citizenship. On both sides, politicians use extreme rhetoric to divide the public into their respective camps. The hardliners, who seem to be winning public opinion in areas along the border, attempt to dehumanize illegal immigrants, much like they do with any perceived enemy during war.
This is done because it's easier for the public to then accept a wide variety of inhumane treatment for this population group. It happened in 1942 when about 110,000 Japanese living in America were brought to internment camps because the attack on Pearl Harbor and war propaganda made them appear dangerous to the public. 62% of those interned were American citizens.