Hindenburg, the 230 duly elected Nazis (with there 13 million voters), and the Nazi party apparatus that made Hitler leader.
If you pull off a political miracle, you might be able to keep a 37℅ plurality (with significant electoral momentum) out of power for a cycle; maybe 2. But a democracy cannot keep such a popular movement out of power indefinitely.
The time to keep the Nazis out of power was the 1932 election. Any move after that is just a Hail Mary gambit