If Hillary were to withdraw, the selection of a replacement would fall to a majority vote of the Democratic National Committee, a largish group numbering in the hundreds of Democrat worthies, such as the chair and vice-chairs of all 50 states.
The current interim chairwoman of the DNC is Donna Brazile, an Oprah-type lady, a veteran of Jesse Jackson’s 1984 campaign and Al Gore’s campaign manager in 2000. She’s widely assumed to be a lesbian. She’s pretty witty and she sort of reminds me a little of Pat Buchanan: highly partisan, opinionated, and combative, but well-liked by other insiders because she recognizes the legitimacy of their right to play the game too. This is her second go-round as interim DNC chair, the previous one had been during the interregnum in 2011 between Tim Kaine and Debbie Wasserman-Schultz.