Nearly every Trump supporter I see says that they support him because deporting illegal immigrants with criminal records is a good thing. Of course it is, but how do you think we're going to manage to do that? Trump can't just say "all illegal immigrants please leave" and they'll be like "okay".
He has no plan for how he's going to approach it, he's just saying what everyone wants to hear. How is he going to do it? Gather up every non-American person he sees in one big group and hand-by-hand look at each person's record to see if they're legal or not, and if they aren't then he'll just ship them off in a helicopter?
No illegal immigrant is going to willfully give themselves up so they can go back to the country they were born in, and chances are they're illegal immigrants because they wanted to get away from there!
He can talk all he wants, if he is elected then everything he says that will happen isn't going to happen. The closest thing he'll get is preventing any Muslims from entering America but that's it.