The problem is that Congress passed the emergency declarations law and now that they don't like the President, they want to prevent him from using it.
The argument I would make is that if a National Emergency can be declared on a long festering issue debated in Congress and a budget bill that Congress designed to prohibit then the National Emergency law is being misapplied. Clearly enough people in Congress didn't think so yet. They can replay it in 6 months while the judges rule on it.