The Republican Party, led by Donald Trump, has promised us a tall concrete wall running along the entire length of the U.S. - Mexico border. Corollary to the wall is to maintain and vigorously enforce existing U.S. immigration law. To do so will be immoral.
If you were born before 1980 or so, you probably remember the existence of the Berlin Wall, or at least its eventual fall. If you are younger or are not very familiar with it, you should click the link and read about it. The East German citizens, under a Soviet puppet government, were told that the wall was to prevent the West Germans from infiltrating their "republic". But the wall's true purpose was to keep East German citizens from fleeing the oppressive Communist regime to a better life in West. The East German government had a reason for doing this. The people leaving were disproportionately young and educated, and this "brain drain" was harmful to the country. Nonetheless, this barrier and the larger Iron Curtain was viewed in the West as evil.
Why was the Berlin Wall immoral? In the Eastern Bloc, people did not have basic freedoms, and they also lacked the economic opportunities that we have in the West. The Wall and the Iron Curtain kept people from freely emigrating from a country with limited opportunity, to one with better opportunities. It kept them imprisoned in a terrible place, and did not allow them to move to a better place. That is why it was immoral. It had a legitimate purpose, but it had an immoral effect, so it was immoral.
Due to differences in economic opportunities between the U.S. and Mexico, many people want to immigrate from Mexico to the U.S. Under current immigration rules, many have no legal path to enter the U.S., so they enter illegally and at great personal risk. Even before Trump, we have already been building a wall on the southern border. Trump's wall will simply be more difficult to circumvent. It will have the effect of keeping people from freely emigrating from a country with limited opportunity, to one with better opportunities. It will keep them imprisoned in a terrible place, and will not allow them to move to a better place. By building that wall, we will be achieving the same thing that the East Germans achieved. We will be no better than the East Germans and the Soviets. We will be committing an immoral, evil act.