Actually, a politician CAN cause economic mayham even before he takes office because investors can see what's coming.
The crash of '08 was due to the country foreseeing that McCain was losing and a socialist was about to take take the Presidency...I have no doubts about that.
3 years before? Well, I don't know...investors may have seen Roosevelt approaching on the horizon (he became Gov. of NYC one year before the crash...wouldn't be surpised if that was a large part of what made the crash inevitable)...or they may just have seen the spectre of a socialist army getting closer and closer.
You are absolutely right. Not only did the passage of Smoot-Hawley in 1930 cause the crash (because it passed a key Senate vote in October 1929, which made it a done deal), but FDR was a Keynesian (because he had read or been told about Keynes’s ideas) long before 1936, when Keynes published “General Theory.”