Harris doesn't need to "attempt it", she somehow became the Democratic choice without a single vote for her, replacing the man who said it'd be ridiculous after all the millions of votes he got
Like it or not (I don't!), primaries are internal functions of membership organizations. The law has found this again and again. In these contests, it is the party delegates who are given the right to choose the party nominee.
Not all coups are military.
https://en.m.wikipedia.org/wiki/Constitutional_coup