From 1938-44 public opinion in Germany was monitored by something akin to 'mass observation'. The survey was headed by professional journalists and they managed to persuade the régime to guarantee confidentiality for all their sources, so one can have some confidence in the results. These have been published and cover 17 volumes.
Hitler was quite popular and his popularity peaked on June 1940 with the very easy German victory over France. After the siege of Stalingrad it fell inexorably.