Trump is obsessed with dominance and humiliation. 

The border wall was perhaps Trump’s biggest campaign promise.

 If he doesn’t get it, it will be humiliating for him–far more than failure to repeal Obamacare, which involves complex legislation out of Congress. 

A wall is a simple budget item. 

If Trump cannot force Congress to allocate the money he wants, he will not take it well. 

There will be a big backlash.