Don't you think fair means, at a minimum, not doing that kind of thing? The DNC's officials were trying to persuade their own party's voters against one primary candidate and for another primary candidate.
As I understand it, a party's purpose is to organize and provide membership for people sharing whatever ideology they share, who are then (among other things) capable of signing petitions for candidates seeking to run under that Party's name. If enough party members from enough districts sign your petition, you're on that party's primary ballot in that State.
Most of what the DNC and RNC do, or what they should do, is either administrative, or attacking the other party. I'm not defending that DNC vs RNC dynamic; I think we'd probably be better off without the party system, at least as it currently exists as a privatized crony-capitalism bullshit theater with very public repercussions... and I'm also in favor of fixing the voting systems used by the States. But, in the meantime, the DNC and RNC should be banned from trying to influence their own primaries.
If DNC officials want to attack one primary candidate, they should quit the DNC and go work for an opposing primary candidate's campaign. They don't want to do that, because they'd lose the status of working under the aegis of the DNC; they'd be working merely for one candidate or another, any of which could lose.
To put it succinctly, a political party official should act in a purely administrative manner with respect to the party's own internal operation, including its primaries.
As an example, say what you want, Donald Trump is clearly not a conservative in any meaningful sense of conservatism. What is the long run result of his candidacy? there is a high probability that the long run result is irreversible damage to Republican party. Surely, party officials can act to prevent something like this, and not simply act as disinterested administrators. A political party in the US is essentially a private non for profit organization - that can run its affairs as the leadership of that party sees fit
That's the entire point.
Having said all that, the democratic primary election was pretty fair and the current candidate is the one who won the most votes.