Opponents of the wall genuinely think that sealing the border is impossible–at least those in the mainstream do. Furthermore, if they refuse to even entertain the notion that sealing the border is possible a) they will never be proven wrong; and b) their adversaries will never be proven right. And it doesn’t hurt that their stance will make them the favored choice at the polls for the very vocal hardcore believers who think that any attempt to close the borders is a betrayal of their ideals.
The argument that X is an intractable problem so we shouldn’t even try to fix it is kind of an odd argument for the left to be making, considering their faith in social engineering. Yet it’s become their fallback position in recent years.
I was wrong about immigration, at least the politics of it. I thought the political divisions would force the competing constituencies into some kind of messy but reasonable compromise that would be an improvement over the current situation. Instead, one side used dishonest arguments and raw political leverage to try to impose its preferred outcome on everyone else, which further radicalized opponents and alienated many citizens who might otherwise have been sympathetic to Bush’s approach.
Whether a real compromise, the status quo or some kind of smaller and more incremental reform is now more likely is anyone’s guess.