Legal means While in prison, Hitler realized that the only way the Nazi's could gain power was to legally attain it. Meaning no violence. The Nazi party would have to be a mass political party that could compete for votes with of other parties.
Popularity Hitler was able to persuade the Germans with his words. He knew what the people wanted to hear so he fed them lies to get their vote on his Nazi Party.
The Great Depression The depression made the extremist parties more attractive to the public. Also with the Great Depression going on Hitler was able to use that as something to say he would change if he was voted for.