What is the #1 responsibility for the US President?

The answer is found in the beginning of his oath at the time of his inauguration.

“I do solemnly swear that I will faithfully execute the Office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States.”

Therefore, any actions we see the President take over the next few weeks are completely justified if he is preserving, protecting, and defending the laws of the Constitution. Anyone opposing his actions is either ignorant and/or an enemy to the Constitution.