I've often wondered about that as well. You'll get a myriad of answers to that question but to me its simply that they were unable to recognise evil so long as it benefitted them.
The scary thing is that in hindsight the evil of Hitler is so clear, but was that so in 1933? Today the evil is clear enough but there are so many that stand to gain from it.
It could win in '08.
In 1933, many members of the middle class were FAR more worried about the Communists, and embraced Hitler as the only candidate who was serious about stopping them. Keep in mind that Germany experienced an attempted Communist revolution a few years earlier in 1919, a bloody Communist revolution occured in Hungary in 1919. Germans in contact with ethnic German relatives in Russia and Ukraine were hearing about the purges, killings, and the Ukrainian terror famine, etc, etc. People were scared enough to elect ANYBODY who seemed a strong leader