Question about the 2020 race

My question is about rules and procedures, not a specific candidate, although it’s triggered by some DUers’ animosity toward Bernie Sanders.

A frequently recurring view on DU is that the DNC shouldn’t have let Bernie run in 2016, because he’s not a Democrat, and should bar him in 2020. This seems to assume that there was a DNC vote to allow him, but I never read about anything like that.

It’s my impression that primaries are governed by the states. Each state with a primary has its own laws about ballot access (filing fee, petition signatures, whatever). The DNC doesn’t control those laws.

The DNC could conceivably decide to play hardball: “We demand that every state adopt legislation to exclude from its primary ballot anyone who’s not a registered Democrat. If any state fails to comply, we won’t seat delegates from that state at the 2020 convention.” That would be seen by many as excessive. Furthermore, in Republican-controlled states like Texas or Ohio, the GOP would be delighted to defy this rule. Then, in races up and down the ballot, the Republican candidates would be saying, “The Democratic Party wouldn’t even admit our state’s representatives at their convention.” That would hurt a lot of downticket Democrats.

There’s also a real question about whether, if push came to shove, the DNC would be willing to punish an entire state. Note that, in 2008, Florida and Michigan violated DNC rules about the scheduling of the primary. The DNC ruled that their delegates would not be seated, then it sort of relented and seated them with half a vote each, then it ultimately caved completely and imposed no penalty.

There are some technical questions about how a no-non-Democrats policy could be enforced. What about candidates from states (like, ahem, Vermont) that don’t have partisan registration? What if a group of registered Democrats want to run for delegate slots as “Unpledged” while letting it be known that they like Bernie or some other candidate whom the DNC regards as unclean? How would caucuses, as opposed to primaries, be affected?

The important issue, though, is whether the DNC could actually do what some DUers keep calling for. Frankly, it seems to me that people are vindictively lashing out at Bernie without thinking through what they propose. Am I missing something? I’d be glad to be enlightened about the mechanics of this idea.