Last month I digested my position on the political externalities of immigration, but I’m happy to elaborate.  David writes:

I could imagine that leading to an additional 300 million people
coming into the United States within a couple of years. My impression
is that Bryan could imagine it too.

I could indeed imagine it, but I think rising rents and declining wages for low-skilled workers would lead to more modest rates.  See New York City: Every American is free to move there, but low real wages and high rents convince most of us to live elsewhere.

David continues:

Now, under my scheme, the U.S. government would couple open borders
with a 20-year residency requirement for U.S. citizenship and a
requirement that one be a citizen in order to get any kind of welfare,
including government schooling… But Bryan and I both know that you can’t always get what you want.
The government is not some entity that he and I control. So what if the
government did not couple open borders with this 20-year residency
requirement or even a 10-year residency requirement? Would Bryan still
advocate open borders?

Definitely.  While I view libertarian rights as prima facie rather than absolute, I do insist on high certainty of very bad consequences before making exceptions.  And the negative political consequences of letting lots of immigrants vote are at worst uncertain, for reasons I’ve previously explained.

David concludes:

And would he worry at all that the new residents would vote away the
goose that lays the golden eggs? And if he wouldn’t worry about that, why wouldn’t he worry? Inquiring minds would like to know.

Of course I would worry.  But mere worry is a poor excuse for depriving millions of their basic human rights to work for a willing employer and rent from a willing landlord.  And I’m confident that immigrant voters would have one extremely pro-liberty effect: Protecting the right of free migration.  As I think David will agree, American voters have sadly shown themselves to be totally unreliable on this vital issue for over a century.