Its not about what he can or can't do, or even how he would do it. He took an oath to uphold the constitution, if he is genuine with his statements regarding what has taken place, if his laywers actually hard data backing their claims then its getting to the point where he either upholds his oath and use the authority we vested in him to protect our society from malicous forces and those that breach the constitution in a manner that hurts the country. Trying the courts and legislatures hasn't produced any significant change in what happened and there's only so much time left be his grip on the executive branch and its offices start to slip past the point of being able to use his position as President to act on what has happened.

I'm assuming its the last resort plan at the moment but the more he let's emotions build up and evedince go nowhere but public the likelier the wheels are to come off regardless of his actions.