It's time that all American progressives realize that open borders doesn't work in a modern welfare state such as the USA.
A high minimum wage for everyone in the USA, universal health care, universal guaranteed housing, universal high quality education, etc. can't happen with open borders on a desperate planet - you'll break the national budget! With 328 million people in the USA and probably at least a billion people that want to move here immediately if they could, there is no way that our economy could absorb all those people in a reasonable, smooth and peaceful way. Only so many jobs paying at least $15.00/hour can exist at any one moment in the economy. And if we were having a recession or depression, that would make matters even worse. Not to mention the massive cultural shock from so many people coming in from very different cultures around the world and the effect this can have on politics and government - just note the right-wing shift in Europe resulting from the relatively small refugee influx there. And, as the Europeans quickly learned with their refugee situation over the last few years, the democratic welfare state can only absorb so many new immigrants/refugees per year economically.
Just think of the catastrophic effect of the addition of hundreds of millions of immigrants on the American environment as they try to obtain the voraciously materialist lifestyle of the Americans that were already here before them. It's ecologically irrational to allow that to happen to the USA and the world in the 21st century.
And another issue, if we had open borders, is that many of the people that would be desperate to move here would be unable to afford safe transportation and would come via human traffickers and many tens of thousands would die as is happening in the refugee migration to Europe (over 33,000 have died at sea crossing the Mediterraen from 2000 through 2017 according to this article by NPR).
Bernie Sanders knows all this - that's why he doesn't support open borders. That's why you've never heard him make a speech in favor of open borders, and in fact during his campaign he didn't say much about immigration in general. That's because as promoter of a progressive future government in the USA, he knows that if you have unlimited immigration, the system will collapse. If you don't believe me, just go back over his speeches - he spoke in favor of helping immigrants in general, but never in favor of open borders.
On the other hand, libertarians support open borders. If you support open borders, then maybe you should join the Libertarian Party and give up on being an American progressive. The libertarian arguments sound reasonable and moral at first, but once you analyze them you realize that they are not rational or practical for a modern progressive welfare state - of course, the libertarians don't believe in the modern progressive welfare state. To see some of the libertarian arguments for open borders see their open borders site here.
You might also enjoy this brief video of the famous economist Milton Friedman explaining why "free immigration" (open borders) is not acceptable in the modern welfare state.
Only when we have something like "The United States of Earth" can we have truly open borders. It will work then because there will be universal welfare, healthcare & other benefits for everyone around the earth and business wealth will be distributed relatively equally around the world, so large masses of people will have no desire to permanently move from one side of the planet to the other to materially improve their lives. Some people will move for marriage purposes, job purposes. for fun and for many other reasons, but it won't be an unmanageable number of people that will feel forced to migrate to another part of the world to improve their lives. But for now the answer is for the wealthy countries to help people in the least developed countries via a 'Marshall Plan' - the way the U.S. helped Germany and Europe become prosperous after the destruction of World War II.
In summary, if you consider yourself to be an American progressive, it's not rational for you to be for open borders at this time in human history on Planet Earth.