

Global population growth is happening. Slowing down, but looks inevitable for at least several more decades. Given that baseline, it is optimal for all countries involved to allow immigration from countries with population growth (reduces strain on government services, adds to the economy with remittances) to countries with lower birth rates (tax revenues support social service budgets, increased entrepreneur rate of immigrants increases job growth, etc.)
Economies can transition to population decline while maintaining standards of living for sure, if handled in a planned way. Some short-term pain during the transition, then fine later. But why go through a combination of short-term pain right now, at the same time as incredible cruelty is required to keep out migrants?
A path to degrowth will be needed globally in the medium-term future (finite planet), but trying to implement that now at just the US locally isn’t going to help the planet at all.

Public support fractures if the questions are broken down into more detail. People have unfounded fears of new “death panels”, and founded fears of the government screwing up implementation (Canada has crazy wait times for many medical services - it’s an outlier among developed countries, but demonstrates the screw-up opportunity). People support new services if they are funded magically, but aren’t willing to support tax raises, even though the tax increases would be less than the savings from not paying for private health insurance.
The complexity - and partisan politicians being more than willing to weaponize confusion over details to divide us against each other - is the barrier.