Hitler's party never won a majority of the vote. They made a coalition deal that made Hitler chancellor, then staged the burning of the Reichstag, just after they removed all the paintings. And used that to declare martial law and seize power by murdering nearly all the other politicians, aided by the Sturmabteilung, an independent militia with it's own aims, including socialism, and a gay leader. The militia were murdered too in The Night of the Long Knives, once the Nazis had built their own new militia, the SS. The background before the Nazis was the weak, polarised murderous and ineffective Weimar Republic, in a country only formed in 1871 & ruined by WW1 & reparations. So, that is far from agreement and complicity.
Similarly with Lenin & Stalin. It wasn't a smooth popular rise. It was a coup by fanatics, followed by ever stricter purges of any opposition.
Trump pretty closely followed the Nazi playbook, but his militia wasn't well organised, and he failed to win over senior military figures. If Trump had succeeded either in keeping power by instituting martial law which many of his team discussed (even Tweeted), or by getting specific state ballots overuled, would that look like universal consent?
Tyranny is probably harder to seize in modern times because it's easier to organise opposition rapidly. But, easier to maintain with tools like monitoring communications and facial recognition.
Jonathan Haidt has interesting research that led him to his moral foundations theory. He has evidence that a sanctity/purity axis is emphasised by times and cultures that feel under threat, such as by neighbouring border conflicts or epidemics. Humans are lastingly impacted by conditions in their late teenage years and early 20s, when oyr brains are still developing.
Democracy is far more fragile than we choose to recognise. Many factors could destabilise it, especially populists amplifying partisanship.
