I work in the construction industry, an industry especially notorious for the employment of illegal immigrants, so I have some personal experience in regards to illegal immigration. First of all, this picture Donald Trump has been painting of violent criminals and Mexican citizens coming here in droves to live off government welfare programs is just not true. These are blue-collar, hard-working, decent people who have come here to escape ruthless violence (fueled by American demand for illegal drugs) and extensive poverty. They have become the foundation that several sectors of the US economy are built on.
What people like Donald Trump and his ilk need to realize is that the violent criminals, for the most part, have no real inclination to pack up and move themselves across the Rio Grande. It's much easier for these drug gangs to operate on their side of the border, where widespread corruption has enabled them to operate with near impunity, than it is for them to "invade" America, where they have to deal with state, local, and federal law enforcement agencies that are more motivated by arrest and seizure statistics than a personal bribe. Speaking from my own experiences, the career criminals that do come across the border end up being arrested over here, which results in their deportation back across the border.
99% of Mexicans here illegally are ordinary people simply trying to support themselves and their families. As things are now, these people live in constant fear that any encounter or interaction with police or government officials will end up with them sitting in a holding cell, awaiting deportation, while their wife and children remain in America. These families are torn apart, with wives and mothers suddenly finding themselves with 3 children to support and no source of income. One woman I know of has resorted to collecting scrap metal to support a 5 year-old son and infant daughter. The sight of a 5 year-old kid helping his mother load an old water heater into a truck so that they might be able to keep a roof over their heads is something that those arguing for tougher immigration enforcement never have, and probably never will, experience.
The truth of the matter is that those working here illegally are not much different from the people who comprise our own working-class. In fact, Mexican workers that are paid "under-the-table" (as in they don't have anything taxes taken out of their paycheck) are relatively rare. Meaning they financially contribute to a system under which they are considered as faceless statistics and under which they have no voice. This has gotten long enough, so I'll stop here. Just remember, that just because someone is labeled "illegal" does not make them a criminal.
P.S.: Before somebody decides to get pedantic with that last statement, I am aware that being here illegally means they are in violation of the law, and technically a criminal. What I was trying to illustrate was the absurdity that people are criminalized for simply providing for themselves and their loved ones.