A lot of American industry, particularly agriculture and food production, relies the cheap and exploited labour of illegal immigrants. Companies that illegally hire illegal immigrants don't have to follow labour laws and can hold the threat of deportation over workers who speak up. They don't have to pay them much or pay much for their safety. Hours are only limited to how much a human being can handle, if even. They make a lot of money this way and donate to the political campaigns of Republican and Democratic candidates. Corporations and other large organizations (like unions) donate to political campaigns to buy influence on policy.
Democracts are lucky that their base is more supportive of immigration than Republicans. They can support their corporate donors in allowing immigration while pretending to do so for altruistic reasons. Republicans, on the other hand, need to appeal to their base by pretending to be anti-immigration. Any Republican policy meant to crack down on illegal immigrants will be token at best.
The mass deportation of illegal immigrants might actually help unions and we all know Scott Walker is no friend of unions. The only person I can imagine who would do less to stop illegal immigration than Scott Walker is Jeb Bush.
