The turning point however was the 1930 election that made them the second largest party. The Enabling Act ultimately led to the formal dictatorship but the Reichstag Fire Decree (also post-election 1933) was what effectively ended civil liberties and political opposition.
By the time the 1933 election happened, Hitler was already chancellor and NSDAP thugs were openly assaulting political opponents. The reason I pointed it out as the "last free election" is that it was the last election that at least offered people the option to vote for someone else.
Starting with the election of 1930 the NSDAP had enough popular support to slowly force its way into dictatorship. Maybe this could have been prevented with a more stable political climate (i.e. a strong majority government and a less fragmented Reichstag) but there's no way around the fact that many millions of people voted for the NSDAP.
This is why every German who's familiar with their history is uncomfortable with the popularity of the right-wing nationalist AfD today. They may not be the NPD but they're certainly at risk of being enablers. This is also why openly anti-constitutional parties can be banned (but also why it's so difficult to do so).
EDIT: Let me reiterate my point for emphasis: While Hitler's rise to power wasn't exactly democratic, the NSDAP was only able to get him into that position because millions of people voted them into the Reichstag. How many people didn't vote for Hitler doesn't matter -- most Americans didn't vote for Trump either.