How do you define “democratic?” Because the “soviet” of “soviet union” is a type of council which was directly elected by citizens. The USSR was a democratic republic in that each soviet usually voted for a higher level soviet. Not that unusual, especially back then.
Now, I’m not suggesting that the books were never cooked. We know that Stalin rigged at least some higher level elections at the very least.
But “democratic” does not mean multi-party. It can also be “no party” or “three parties” or anything. In the USSR you could run for your local soviet or petition them to vote for you. Yes, you’d have to be a party member. But that doesn’t mean blind allegiance and no differing thought. I’ve brought it up before, but you had severe infighting in the party because of the diversity of opinions and thought, not lack of it. Sure, they were all communists or some flavor thereof at least superficially. But there’s a hell of a difference between Stalin and Kruschev and Gorbachev as examples.
And, Stalin aside given his prominence in the early years of the nation, the other prominent leaders were very dependant on entities like the Supreme Soviet which was elected by your elected representatives.