The correct answer to this open question is the following.
The President is the leader of the political party of which he is a political party leader.
As it is stated in the question, the President of the United States has many roles to perform. He is the leader of the nation, the Head of the government. He is the Commander-in-Chief of the armed forces(Navy, Army, Air Force, and Coastal Guard). He also is the Chief Diplomat of the nation. But in he is also the one that likes to dictate its political party the important topics that have to be discussed in Congres. That is why he is considered the political party leader. His party has to support all the presidential initiatives that are considered for the benefit of the American people.