The DNC is pretty horrifyingly corrupt.

If you're a self-described centrist, moderate, independent, or swing voter, you might have a very difficult time understanding why Sanders is so vehemently against the Democratic party establishment, and why his voters are so angry with the DNC and with prominent Democratic elected officials. You may even think they are paranoid, or conspiracy theorists, or simply outright extremists. I address (and, in limited ways, cede) those arguments elsewhere on this site.

Then again, you might not. There are good reason to think that many independents distrust the two established parties and find fault with both the DNC and the RNC orthodoxy, so maybe some centrist voters actually sympathize with Sanders' criticisms. (Of course, there are also many soft independents who when pressed are much closer to one party than another.)

Let me first say that there is dishearteningly little journalism about the DNC and RNC as public institutions that are accountable to journalistic and voter scrutiny. In the U.S. we seem to obsess over candidates, and party insiders are very much that -- inside. They are far less visible to the public than in most European political parties, where a press conference with party leadership or a summit of regional party heads is front-page news.

Here are my arguments why the current DNC is far more corrupt than most people understand or imagine.

1. They accept corporate money. Lots of it. And the companies they take funds from include hedge funds and investment banks. In a two-party system, the Democratic party allows itself to owe favors to actors in the financial sector that are the source of systematic abuse and reckless practices, hindering its ability to regulate the economy capably.
2. President Obama, to his credit, banned donations to the DNC from lobbyists registered with the federal government and from PAC's (political action committees) in 2008. That ban stood until 2015, when Debbie Wasserman Schultz, chair of the DNC at the time, lifted it. The DNC went backwards since Obama, expanding corporate influence over the party instead of limiting it. The official platform of the DNC at the time called for "immediate action to curb the influence of lobbyists and special interests on our political institutions." In other words, committed Democrats can put a nice platform together, but it's meaningless if Debbie Wasserman Schultz simply ignores it, indeed actively undermines it.
3. The DNC allowed itself to be nearly totally subsumed into the Hillary Clinton presidential campaign starting in early 2015 -- at least a full year BEFORE the first votes were cast in the nominating process, which Senator Bernie Sanders entered after Clinton had cleared the field for herself. The Clinton campaign hammered out a joint fundraising agreement with the DNC which was so tilted in Clinton's favor that she had direct say over how funds donated to the DNC would be spent. The agreement dictated the money go towards purposes like opposition research on Republican candidates and certain investments in DNC infrastructure, instead of being available at the discretion of DNC leadership, which could and should have put the funds to better use, such as voter outreach, a 50-state strategy that would have ultimately helped Clinton in Wisconsin, or money to vulnerable down-ballot candidates in state legislatures at risk of flipping parties. What's worse, during the active primary process that still involved Sanders as a candidate, the Clinton campaign began redirecting that money into its own campaign coffers. It's hard to see how Sanders even brought himself to make peace with DNC in 2016 (which he did, admirably, wholeheartedly, for the sake of trying to defeat Trump) after that level of collusion and corruption at his expense.
4. Al Gore's 2000 campaign also demanded control over DNC spending well before he was the nominee, while the primary campaign against Bill Bradley was ongoing. Apparently, it is standard practice for the DNC to crown a nominee internally long before voting is even finished. If the RNC had used similar tactics in favor of Romney in 2011 or Jeb Bush in 2015, their voters would have revolted immediately. (Of course, today's RNC is so extraordinarily corrupt that they are no longer a useful reference for matters of fairness or transparency.)
5. DNC chair Debbie Wasserman Schultz resigned after hacked emails proved a bias against Sanders among party insiders.  
6. Her replacement, Donna Brazile, leaked questions to the Clinton campaign that CNN, Brazile's employer, was planning to ask Clinton and Sanders at an upcoming debate. Brazile was still allowed to run the DNC after this incident. CNN fired her for this, but the DNC made her its chair.
7. Tom Perez' sole purpose in running for DNC chair after Brazile was to serve as a roadblock to Keith Ellison, a Sanders supporter.
8. At the election among DNC members between Perez and Ellison, Brazile changed the rules that the party had explicitly agreed upon in its bylaws that the ballots were open, and single-handedly, with no justification and with no scrutiny from the press, made the ballots secret. This likely allowed DNC members to support Perez' candidacy even though they had pledged support publicly to Ellison. On a second ballot, more DNC members switched their votes from Perez to Ellison, again, in violation of the DNC's own rules.
9. When the Sanders campaign sued the DNC in federal court for its institutional bias against his campaign in favor of Clinton's, its defense was that it had a right to do so, since neutrality is only a "discretionary rule that it didn't need to adopt to begin with". In other words, they argued in federal court that even if they had made their own internal rules about being fair, they were not bound by law to actually follow them.
10. They went even further and argued that they were not accountable for any grievances raised by Sanders supporters, who claimed that their rights were violated by voting in a biased primary: DNC lawyers argued that "the Court would have to find that people who fervently supported Bernie Sanders and who purportedly didn’t know that this favoritism was going on would have not given to Mr. Sanders, to Senator Sanders, if they had known that there was this purported favoritism." In other words, Sanders supporters all knew that it was rigged against them, so they weren't cheated; they only would have been cheated if they had given Sanders money not knowing that he was being screwed openly screwed by Clinton-affiliated DNC officials.
11. Tom Perez has stacked the DNC with corporate lobbyists and people whose very source of power is at odds with the DNC party platform as it is written. Now remember, Perez was Obama's Secretary of Labor. So who does he appoint to the DNC? Union leaders? Organizers? Lawyers who fight wage theft? University economics professors? Nope. Corporate lobbyists.
12. The DNC is allowing candidates to directly employ these Superdelegates. So what has Michael Bloomberg done? Gone on a hiring spree. If the Bloomberg campaign paid me a healthy salary, I'd probably be inclined to vote for him as a Superdelegate, wouldn't you? Shouldn't there be, I don't know, some kind of rule against this? Dear DNC, what in your policies would forbid Bloomberg (or any candidate) from just straight-up buying Superdelegate votes? If George Soros gave $10,000,000,000 to Sanders for the sole purpose of employing as many Superdelegates as possible as well-paid consultants, might you then shut down this kind of corruption?
13. At least one of the DNC's Superdelegates donates money to the RNC as a healthcare lobbyist. As reported in The Intercept, "I am a committed Democrat but as a lobbyist, there are times when I need to have access to both sides and the way to get access quite often is to make campaign contributions." Translation: "I support abortion rights for women and marriage equality for gays and lesbians, and I think the EPA should exist, so I'm a Democrat, but as for healthcare, I am fine with it being expensive and I am fine with the fact that access to it is denied to millions unless my company profits. Also, I'm willing to bribe politicians in either party, so that healthcare stays expensive and so that it is denied to millions." This same Superdelegate, William Owen, openly expressed that he is willing to help keep Sanders off the Democratic ticket for president, even if Sanders wins the most votes in the nominating process from voters.