I think Trump will get his wall either:
1) By having them accept the compromise. Upside, they get something they want. He's been giving them carrots, but no nibbles.
2) Declaring national emergency, and the funding the wall from defense budget. Which should be easier, since Trump has stopping giving away all of our money to foreign aid and has been cutting back on the the whole "Team America World Police" act. Of course, this is the stick, not the carrot. But, in this scenario, they get jack and squat. Moreover, he can just decide to be a dick to them from then on by vetoing every single bill they submit to him and lose nothing, since there is no partnership to "destroy".
Option 1 is still more favorable for them, whether they are smart enough to figure that out or not is often a question I ask myself frequently.
If the Democrats force Trump into option #2, they will lose 2020. The reason being is the vast majority of Democrats want various elected officials to work together, and even they, at this point, know about the carrots. There is no way they're going to spin this as Trump's fault, and Trump still gains points keeping his election promises no matter how he gets there with his base -- even if the actions are seemingly extreme, no one will fault him for taking the nuclear option if he's backed into a corner.