Shouldn't that be an easy question to answer though?
If a supermajority left-wing government changed immigration law to free and unrestricted passage across borders, making anyone who sets foot on US soil is legally a resident if they wish and further entitled to a pathway citizenship if they want it, then that would be the law and must be followed.
Anyone would still be free to run their campaign on changing immigration law back, or to something else. Economic and societal performance under that hypothetical law change would determine if a supermajority of "change immigration law to XYZ" then gets elected to do that.
There is always a possibility that putting no or too few limits on immigration causes irreversible damage to a country before course correction can happen, but the same is true for extreme polarization and unresolvable political divide.