All the Democrats achieved by resisting the bully's border demands was some virtue signaling. They gave in on the substance.

Lawsuits may stop the bully from using a "state of emergency" to get more money for the wall, but even if that happens, the bully has already won a victory. If he does the same thing twice a year, he will get most of what he wants by the 2020 election.

This demonstrates the error of negotiating by making sticking to the status quo your demand. Your adversary can easily suggest making a "compromise" where you give up a part of what it wants. Each time this happens, you lose.

If you have made a deal, and your adversary demands to change it, you should respond by demanding a change in the opposite direction. Say something like this:

The previous spending levels do not represent what we stand for. They are a compromise: we made concessions to get that agreement.

If now you tear up that compromise and demand more, we do the same. In response to your demand for A, B and C, we now demand X, Y and Z.

After that, the natural way to meet each other "half way" is to stick with the status quo.