Democracy fails when people elect regimes that are hostile to basic human rights. This has happened a number of times. Some cases, like Hitler's election, involve intimidation. But just as often, a tyrant can gain power by promising the electorate something it wants, typically nowadays a pipeline into other people's pockets.

Keeping pure democracy from self-destructing is exactly what a Constitution is supposed to accomplish, by setting certain basic principles above simple majority rule. Of course, just having a Constitution isn't always enough. A lot depends on how it is written and what its purpose is.

Venezuela has a rather extensive Constitution.

So did the Soviet Union.