Not necessarily because there were quite a few who were anti-45ers. Remember too that we have a "primary system" where party members compete against each other for the job... and there WAS a primary, and he eventually outlasted all of them, including his closest competition - Niki Haley.
But because he was the last one as "President", there was a de facto clinging to him for party identity.
But in your scenario, the last one who WAS "President" among the Democrats was who? 
Joe Biden.
And he would have been considered "the leader". Harris only ran at the top of the ticket (she had been on Biden's ticket) because he dropped out and she received the majority of the votes in a hastily-called delegate vote ahead of the DNC Convention (since the Democrats didn't really have a primary given Biden was the incumbent... outside of brainworm's attempt to shoehorn his way in (before he switched and tried to get his name off of ballots)).
And I wouldn't say she "disappeared". I think it was more a reaction to a mass rejection by some constituencies who had previously voted for Biden, who later found out about FAFOing, and her trying to do some re-calibrating.
I know here on DU, there are many who are lukewarm about her as threads about her often get yawns.
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