Sebastian Haffner wrote a book which was published posthumously, as he had abandoned the project in the 30s. He describes how he and his friends saw very clearly the absolute danger in the "quadrupeds" and "barbarians" whose "spirit emanated from toilets in military barracks and execution yards". Many critics of the Nazis were assassinated or harassed, be they normal communist workers or intellectual writers. The left-wing press strongly agitated against the Nazis. Thomas Mann once was shouted down in 1930 when he gave a speech entitled "German Address. An Appeal to Reason", where he criticised the Nazis three years before they came to power. Here's an excerpt from his son's autobiography, where he unexpectedly ends up sitting at a table next to Hitler's:

It was certainly not a pleasant feeling to sit in the vicinity of such a creature; and yet, I could not stop staring at the repulsive face. I had never found him especially attractive, neither in pictures, nor on the illuminated stage; but the ugliness opposite which I now found myself still exceeded all my expectations. The vulgarity of his features calmed me. I looked at him and thought: you will not win, Schicklgruber [Hitler's father's original name], and if you cry your soul from your body. You want to rule Germany? You want to be dictator? With that nose? You make me laugh (...) you will never see power.

Was there no bloody halo around his head to warn me? No writing on the wall of the Carlton tea room?

Remember, it was a long period from Hitler becoming a party member (1919) to taking power (1933) to formally abolishing democratic rights (also 1933). The Nazi party's fortunes rose and fell until the economic crash, the inept crisis management of the elites and the elite's support for any right-wing organisation (out of fear of the communists) started Hitler's final rise to power. During all that time, many people warned and some foresaw the disastrous magnitude of his evil. It wasn't enough.