The Democrat Party's official position is that they do NOT want a contested primary. Such a primary would expose the participants to uncomfortably debate issues they desparately want to keep swept under the rug, or just as importantly, give the mainstream media a huge headache in how to APPEAR to be asking tough questions on the candidates. The media solution would be easier, in so much all they have to do is to structure questions as "how can your (the candidate's) policies effectively negate the mean, oppressive GOP's position on this issue" or something to that effect that promotes the positive while suppressing the negatives out of the debate.
But a debate WILL necessarily expose Hillary's (and in extension, the Democrat Party's) vapid positions on national defense, the economy, and virtually every other important political subject. So rather than say that DU is endorsing Hillary Clinton, the REAL issue is they want to suppress any Democrat dirty laundry. Period.