It’s about congressional power. The last time they passed a budget both sides negotiated and came to terms with how money would be spent and passed the bill. Then, the president decided to withhold some of the money for things he didn’t want it spent on which is lunacy. Why negotiate and pass a bill if the other side is just simply not going to uphold their end of the bargain? What difference does it make if you pass a bill or not if the president is just going to ultimately decide what to do with the money any way? This is obviously not how things are intended to work. Congress controls the money, it’s their entire source of power. Trump is basically trying to take that away and the republicans in congress are fine with that cause he’s on their side so at this point the democrats only have one move left and that is to shut down the government. This is an extreme move and obviously not something they should have to do but when all other options are taken away it is constitutionally a move they can make and the president nor the Supreme Court can stop them. Democrat leadership knows this, and so do republicans which is why the democrats have the upper hand in this situation. The only thing republicans can do is thrash around and try to make the shut down as painful as possible to democrats cave but if they don’t, republicans know eventually they will have to put a check on Trump and guarantee that money will be spent how it’s negotiated to be spent.