I came across Inside Hillary Clinton's Secret Takeover of the DNC and it's interesting. Donna Brazile is promoting a book about the inside game at DNC.

"When I got back from a vacation in Martha's Vineyard, I at last found the document that described it all: the Joint Fund-Raising Agreement between the DNC, the Hillary Victory Fund, and Hillary for America.

The agreement--signed by Amy Dacey, the former CEO of the DNC, and Robby Mook with a copy to Marc Elias--specified that in exchange for raising money and investing in the DNC, Hillary would control the party's finances, strategy, and all the money raised. Her campaign had the right of refusal of who would be the party communications director, and it would make final decisions on all the other staff. The DNC also was required to consult with the campaign about all other staffing, budgeting, data, analytics, and mailings."

Essentially, the Clinton campaign bought the party in August 2015.