I answered this in the other thread, but I’ll copy it here too —
There is no formal leader of a U.S. political party. If the president is from that party, then the president is the de facto leader. But if not, there’s no designated leader.
The head of the national committee can play that role if he or she is motivated and effective. But the national committee is not the party. Very often, the highest elected official from that party will be looked to to provide that leadership.
American parties are very different from, for example, British parties. They’re not true organizational entities. They’re loose coalitions of individuals and organizational entities that have chosen to wear the party label.
To the extent that there’s any leader, it’s because some substantial number of party members have chosen to treat someone as a leader.