when it comes to politics, there is a difference between opposition and indifference, and i think this is sometimes lost or exaggerated in the discourse.
personally, i would be opposed to climate change, but indifferent to "religious freedom". i would be opposed to poverty, but indifferent to inequality. so, i can vote for a party that is pro-muslim (to a point), so long as it rigorously opposes climate change. and, i can vote for a party that is broadly bourgeois, so long as it puts some redistributive policies in place to address the worst kinds of poverty. conversely, i wouldn't be interested in voting for a party or candidate that demonizes the rich, but doesn't believe in wealth redistribution.
i am not an american, but i do not find myself particularly opposed to a border wall. rather, i find myself largely indifferent to it. and, i might suspect that this is the case for most voters.
it is true that there's not much of an argument in favour of the thing, but it is also true that there's not much of an argument against it, either. give it a try. try and argue against the wall, convincingly. the best thing you're going to come up with is the cost of the thing, but that's a bad argument: infrastructure projects have high multiplier effects and are good for the economy.
as there is really little reason to oppose the wall to any great extent, i don't expect that the democrats are going to be able to hold support much longer, if it isn't already starting to crumble. the initial kneejerk reaction may be to blame trump. but, the longer this goes on for, the more it hurts the democrats.
trump's base is large nihilistic and doesn't care about the government, anyways. he's not going to lose support over this. but, the democrats are risking a dangerous backlash if they wait too long to cave.
i'm not going to say to build the wall. i'm indifferent to it.
but, i am going to remind people that they need to pick their battles.