Most national emergencies have been sanctions laid against nations or individuals. It'd be foolish to think there wasn't a political agenda in that, but that's more of an international political agenda (that both parties would support) rather than a domestic party-line agenda.
This is the first time a president has ever asked for something, been denied, then declared a national emergency to get it anyway. It's childish, sure, but it sets a very dangerous executive precedent. It basically opens the door to allow the president to do anything they want, with military spending, without checks or balances. I'm not saying Trump plans to become a dictator (he doesn't seem to plan really anything), but this action would open the door for someone else who does.