One of the problems with this debate is that half of the problem is ignored much of the time. It is always framed as keeping terrorists out. Of course that is a valid concern but it is not the only one and probably not even the main one.
Most countries do not allow mass immigration. Russia does not allow two million Australians to just waltz their Matildas over the border. Of course we have nothing against them and we know they are not coming to try to kill anybody. They are generally welcome to come legally in small numbers; but only in small numbers.
All of this must be financed. Resources are finite. No country, not even the USA has sufficient resources to feed, clothe and house infinite numbers of people. There must be a limit.
Therefore anyone who enters illegally is denying a place to a person who has come through the proper channels. And Americans must understand that if illegal immigration is tolerated then their will be enormous tides of it.
Germany is already finding out the hard way that you cannot have a social welfare system and a free health care system if you also have open borders. It is simply not possible to maintain both. People will migrate towards those countries with the most generous benefits. I don’t blame them; but it is not sustainable. This is not about party politics or left / right divides. This is just common sense.