More like Congress is an easy mark for Trump. He sent the Congress a letter on Friday invoking the 1985 Balanced Budget and Emergency Powers Act or whatever they called it. Anyhoo Trump can declare a national security emergency on the border and take DOD money to start building the wall. He can also choose not to spend any portion of the Omnibus money and simply return it to Congress. There is nothing they can do about it except start a lawsuit which will happen. Then it goes to SCOTUS where Trump will easily win due to separation of powers. That’s why he is announcing that they will start the wall on Monday.
Overall, I still prefer him to most alternatives, but folks will be less disappointed if their expectations aren't unrealistic. Everything is negotiable, right down to his signature issue, the wall.