See my last paragraph above. Plus, he knows that if he goes with the declaration, there's no going back, with Chuck-N-Nancy doubling down on The Resistance. He keeps threatening it because he's hoping Chuck and Nancy will be able to care more about the safety and security of the American people than they do about making sure Trump doesn't get a win to use for reelection.
What you call The Resistance, I call the mid-term elections in which the American people turned out in record numbers to take control of the U.S. House from the Republicans and give it to the Democrats. They did not do so because they were pleased with the results Trump and the Republican Congress were generating. They did it because because they were displeased.
If it's about caring more about the safety and security of the American people, Trump has mishandled the politics of that in a way from which he may not be able to recover. We just completed a month plus in which the news presented every day man-on-the-street, your-neighbor stories of people who were hurt by the shutdown. Polls show Trump is blamed for this pain far more than the Democrats.
When reports came in about people being delayed at airports because of the shutdown, the pain became too great for even the Senate Republicans to bear. That's the point McConnell told Trump he could not longer hold Republican votes in support of Trump. And that's the point Trump, not Pelosi, made the big announcement about ending the shutdown, receiving NOTHING for his troubles and the trouble he caused for the Republicans in the Senate who can cast the decisive votes to impeach him.
With the Democrats now in control of the House, this is the first time Trump had the true chance to demonstrate the deal-making, get-things-done skills he claimed to have when he campaigned to be president. If conservative commentary is any indication, what Trump showed did not impress his base.
And what did Pelosi and Schumer have to do to achieve what is for them such a politically wonderful result? They simply said no to wall funding and sat back while Trump did all the rest. Because that strategy worked so well, look for more of the same in the future.
Again, this is the first time Trump has faced true opposition in Congress. His now-empowered opponents know the political game better than Trump and it shows.
If Trump declares a national emergency, it will not be for him a way out. It will be another opportunity for the Democrats to trip him up in a web of his own making.