The Clinton and Zero it is pretty simple. Bring in a 3rd world population and you have yourself voters looking to DC for money. I think Reagan was fooled with the idea that there would an amnesty in exchange for enforcement which never happened. Bush I was a chamber of commerce Republican who loves illegals for the cheap labor. Bush II bought into the Rovian lie that he should push amnesty to secure the Latino vote.
Those all seem pretty shallow, given that almost all the rest of us see a serious threat.
I wonder if the reasons run deeper than that. I don’t think this sort of long-term slow invasion has a historical precedent.