The Left just can't wrap their heads around what's going on...
As previously marginalized groups gain power, that power must be taken away from straight white males. Naturally, the folks losing power are not happy about it. They feel ignored and rally behind the guy who makes them feel heard.
It doesn't matter if they are right to feel that way. It doesn't matter if "reverse racism" is wrong or right. It doesn't matter if the "invasion" is really just a group of poor people consisting mostly of women and children. It doesn't matter if manufacturing jobs aren't ever coming back, no matter how much the immigration policy is tightened.
These people have these concerns, and the Left is not addressing them. Period. Trump is addressing them. That's all that matters. Trump's genius lies in his ability to market himself to stupid people. So the Left needs to get better at marketing their policies to stupid people.
The biggest demographic gap in Left/Right support is now college educated white women vs uneducated white men...all because the Left is unable to sell themselves to stupid people. Trump's masterstroke was to rile up his base with the "invasion" and the Kav hearings to ensure they turned out to vote. Pure genius at engaging the stupid masses on display by Trump.
When your 3 year old is scared of monsters under the bed, you don't tell the child they are stupid monsterphobes. You make a silly thing like "monster spray", spray it under the bed, fix the problem as far as the child is concerned, and then go about your business taking care of real issues.
So build the stupid wall. Who cares? Everyone with half a brain understands it is pointless, but it appeases the middle America morons with the intellect of children who will never even travel far enough from their little home town to see the silly wall. Then, when you have them on your side, you can accomplish things that will really benefit them...like free education to prepare them for jobs that can't be taken over by robots, or clean energy, or healthcare, or all the other things they vote against because they are afraid of monsters under the bed.