If someone wants to work at a job, and someone else wants to offer that job to said individual, what right does anyone else have to intervene between the two? That's no different from stopping two people who love each other from marrying. What business is it of yours? What right do you have to dictate or regulate between the two interested parties?
In a country offering welfare, unlimited immigration of anyone who feels like coming over here and taking our welfare spending -- and by this I mean any public spending, even if it's on cops, schools, hospitals or whatever, not just food stamps or temporary assistance to needy families -- would obviously capsize and sink the entire ship of state. There are billions of poor, starving people who would do anything to come here and accept all the generous benefits they would immediately start receiving upon entry. Therefore, open borders cannot coexist with the modern state as we know it, which has been around since the 1930's and the whole developed world is pretty happy/content with. No one wants to scrap their public schools, public health care, social security benefits, etc. Open borders only works within the context of anarchy, where the state offers no assistance to the individual, so coming here doesn't bring anyone any benefit at all above where they used to live.
However, this restriction doesn't matter if the immigrant has already secured his or her upkeep ahead of time. If they have a job or are married to someone with a job or the child of someone with a job, the entire issue of 'sinking the ship via overcrowding' evaporates away. In fact, since only 60% of Americans currently have a job, importing a batch of legal immigrants who have a 100% employment rate would not only not sink the ship of state, it would provide additional buoyancy and solvency to our tax rolls and ability to spend on the indigent. There is no economic argument possible against importing any number of people who want to work here and can find work, it's pure upside for both individuals involved and for the state's budget as a whole.
Cultural arguments make sense in homogeneous nations that want to upkeep their own unique heritage and traditions. I don't think Japan should be handed over to Filipino immigrants even if they could all find work in the country, because then Japan would cease to be Japan, which is currently the greatest nation on Earth. But America has no heritage and it has no traditions. Most of the children born in America aren't even white. The country was only founded a few hundred years ago, from a polyglot of immigrants from all around the globe. America was just a blank slate for people to come here and find work from the beginning. It always has been and it always will be. Since there's nothing worth preserving about America anyway, it doesn't really matter who comes and lives here next.
As for political arguments, I don't see how the voting record of 100% employed immigrants would automatically be worse than the current inhabitants of the country. They'll probably be moderates who don't rock the boat either way and just concentrate on earning a living, based on the record of all previous waves of immigrants to America. Since over 50% of American children are nonwhite, the Democratic party is destined to take over anyway (in fact, I'd argue it already has taken over a permanent hold on power), so ushering that era in sooner via massive legal immigration really wouldn't make any difference.
If it's true that robots are going to shortly replace most low-skill labor, such that importing a bunch of currently employed but ultimately unemployable citizens would be an error in foresight, this really doesn't matter anymore because we've unlocked the cornucopia of free, robot produced goods which can then turn around and give everyone free money, including the newly unemployed.
It isn't true that America is overloaded environmentally. Most of America is still empty, and the country is severely underpopulated compared to comparable other regions, like Europe, Japan, China, India, etc. There's no reason why we couldn't host a billion or three billion people within our borders. There's always Alaska, which is about as big as the rest of the U.S. combined, and with global warming on our side it should soon be as toasty and welcoming as Idaho/Oregon/Nebraska/Wisconsin/Massachusetts etc. If people can live in all those places now, one hundred years from now when temperatures are much higher, Alaska will be just as good as them and a lot more vacant.
There is no rational reason why America shouldn't become a 'nation of immigrants.' It's immoral to shatter people's lives for no reason. So legal immigration should no longer have any limits, but simply be a job buffet. Anyone who can line up a job to work in, in America, can move here and become an American citizen, no questions asked. Rich, poor, middle class, whatever. So long as you can line up the job, you've proven your worth, and in you go.
Meanwhile, illegal immigrants who have been living here for decades should be given amnesty and citizenship already. It's just a joke to keep people who have spent the majority of their lives in America, and who are well integrated into our neighborhoods, communities, churches, schools, jobs, etc, 'living in the shadows' with no legal right to even exist. Most illegal immigrants have family members who are legal immigrants or US citizens. In order to deport the illegal immigrants, you would have to tear these families apart, a clear violation of human rights. And no, it doesn't work to just say 'oh, we'll just deport the legal immigrants and the US citizens too!' First off, there's no legal way you could do that. Second off, there's no way they would volunteer to go back with their illegal brethren, because their jobs and their financial security are here in America. Their kids have friends in American schools. They have friends they don't want to be separated from. If they went back to their nation of origin, they'd no longer even be able to support themselves and they'd probably all starve to death on the side of some road in a ditch, because they left those countries due to the lack of economic opportunity to begin with. In any case, they will certainly experience far more hardship moving back to whatever hellhole they crawled out of than if they just stay in America. Which means, any rational person will stay in the USA even if their relatives are deported. Which means, yes, you really are separating families against their will by deporting illegal relatives of legal immigrants and US citizens.
Bullying weak, defenseless poor people who haven't done any crimes in our country for decades, but have just wanted to work and make a living, is immoral. They've already earned the right to live here by working and not being criminals. Just give it a rest already. They're a part of America now, after decades of living here and planning their future around living here and having their children who are full US citizens here, it's far too late to turn back the dial now.
An amnesty of all current illegal immigrants should be accompanied by border security to stop all future illegal immigration. With lax legal immigration that has no upward limit save the requirement that you find employment first, there should be no reason why the 'right' type of immigrant cannot come here legally. Therefore, we know for a fact that anyone attempting to cross our border illegally is the 'wrong' type of immigrant we do not want. They're likely terrorists, gang members, criminals, welfare seekers, have ebola, or whatever. We should secure our borders and make sure the whole world understands that the only immigrants we want coming here from here on are people who have lined up a job and will be an asset to the country, not a burden. Obviously, this means scrapping other immigration visas we've previously allowed, like the insane 'diversity' visa, family reunification visas which end up chaining every last member of a country into legal citizenship despite none of them being economically qualified to succeed here, refugees fleeing supposed or even real persecution abroad, etc. If you can get a job, you're in. If you can't get a job, too bad. Go look for some other sucker. I hear Sweden will accept anyone with open arms.
America's immigration policy could be anything, with the right domestic policies. We could deport everyone except blond haired blue eyed IQ 150 supermen and name ourselves the Aryan States of America, and ban anyone from coming in and polluting our perfect new gene pool. This is not a bad immigration policy, but it's doubtful the American public would accept it, so it's not really worth discussing. Likewise, a libertarian anarchy with absolutely no public spending whatsoever could have a rational open borders policy with no restriction on who can move here, but Americans are fond of their public spending programs and aren't going to give them up, so an open borders immigration system won't work either. Accepting only valuable additions to America while accepting the fait accompli of illegal immigrants who are already living in our country is a viable compromise the country could actually unite around, and thus is an option worth discussing.
The country wants border control. They don't want poor people or criminals just flooding in and taking advantage of America's prosperity. But they also don't want to bully or harm innocent people who just want to make a living. No one is going to accept a policy that rounds up women and children at gunpoint and marches them across the southern border, or through various rental and employment measures puts them in a situation where they're homeless and unemployed, and simply to avoid starvation and death by exposure to the cold flee back south on their own initiative. In a country with $53,000 per capita, we can afford to be a little more charitable than Eskimoes were with their grandparents. This type of blood curdling enforcement may have worked in B.C. times, but at the very least once Jesus entered the picture the American public is no longer fond of such solutions. We're going to have to keep our borders open, and we're going to have to forgive the illegal immigrants who have already come here, because we won't be able to look ourselves in the mirror if we don't. Morality dictates a near-total surrender on the immigration issue. Making a reasonable compromise while we still can, and Republicans have at least some say in the government, is a better idea than just waiting for the Democrats to gain total power (and they will, demographics is destiny) and unilaterally totally opening up the borders to all comers.
Fix immigration now, while we still can. It will only get worse from here.